diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts b/packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts
index a2e8e53ecb..482cd44a0e 100755
--- a/packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts
@@ -53,6 +53,8 @@ async function compileFixture({ input, dirname }: { input: string; dirname: stri
             // TODO [#3331]: The existing lwc:dynamic fixture test will generate warnings that can be safely suppressed.
             const shouldIgnoreWarning =
                 message.includes('LWC1187') ||
+                // TODO [#4497]: stop warning on duplicate slots or disallow them entirely (LWC1137 is duplicate slots)
+                message.includes('LWC1137') ||
                 message.includes('-h-t-m-l') ||
                 code === 'CIRCULAR_DEPENDENCY';
             if (!shouldIgnoreWarning) {
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/error.txt b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/error.txt
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/expected.html b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/expected.html
new file mode 100644
index 0000000000..120c3cbe3c
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/expected.html
@@ -0,0 +1,23 @@
+<x-outer>
+  <x-inner>
+    a
+    <!---->
+    <div>
+      yolo
+    </div>
+    <!---->
+    b
+    <!---->
+    <div>
+      yolo
+    </div>
+    <!---->
+    c
+    <!---->
+    <div>
+      yolo
+    </div>
+    <!---->
+    d
+  </x-inner>
+</x-outer>
\ No newline at end of file
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/index.js b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/index.js
new file mode 100644
index 0000000000..17a015b5ea
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/index.js
@@ -0,0 +1,3 @@
+export const tagName = 'x-outer';
+export { default } from 'x/outer';
+export * from 'x/outer';
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/inner/inner.html b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/inner/inner.html
new file mode 100644
index 0000000000..f1edc5794c
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/inner/inner.html
@@ -0,0 +1,9 @@
+<template lwc:render-mode="light">
+    a
+    <slot name=foo></slot>
+    b
+    <slot name=foo></slot>
+    c
+    <slot name=foo></slot>
+    d
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/inner/inner.js b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/inner/inner.js
new file mode 100644
index 0000000000..2104845f74
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/inner/inner.js
@@ -0,0 +1,5 @@
+import { LightningElement} from 'lwc';
+
+export default class extends LightningElement {
+  static renderMode = 'light'
+}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/outer/outer.html b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/outer/outer.html
new file mode 100644
index 0000000000..1ae6aeda57
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/outer/outer.html
@@ -0,0 +1,5 @@
+<template lwc:render-mode="light">
+    <x-inner>
+        <div slot="foo">yolo</div>
+    </x-inner>
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/outer/outer.js b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/outer/outer.js
new file mode 100644
index 0000000000..2104845f74
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/light/modules/x/outer/outer.js
@@ -0,0 +1,5 @@
+import { LightningElement} from 'lwc';
+
+export default class extends LightningElement {
+  static renderMode = 'light'
+}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/error.txt b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/error.txt
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/expected.html b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/expected.html
new file mode 100644
index 0000000000..7296bb8ba4
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/expected.html
@@ -0,0 +1,21 @@
+<x-outer>
+  <template shadowrootmode="open">
+    <x-inner>
+      <template shadowrootmode="open">
+        a
+        <slot name="foo">
+        </slot>
+        b
+        <slot name="foo">
+        </slot>
+        c
+        <slot name="foo">
+        </slot>
+        d
+      </template>
+      <div slot="foo">
+        yolo
+      </div>
+    </x-inner>
+  </template>
+</x-outer>
\ No newline at end of file
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/index.js b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/index.js
new file mode 100644
index 0000000000..17a015b5ea
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/index.js
@@ -0,0 +1,3 @@
+export const tagName = 'x-outer';
+export { default } from 'x/outer';
+export * from 'x/outer';
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/inner/inner.html b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/inner/inner.html
new file mode 100644
index 0000000000..81aec27aa4
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/inner/inner.html
@@ -0,0 +1,9 @@
+<template>
+    a
+    <slot name=foo></slot>
+    b
+    <slot name=foo></slot>
+    c
+    <slot name=foo></slot>
+    d
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/inner/inner.js b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/inner/inner.js
new file mode 100644
index 0000000000..a12e9aa6b0
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/inner/inner.js
@@ -0,0 +1,4 @@
+import { LightningElement} from 'lwc';
+
+export default class extends LightningElement {
+}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/outer/outer.html b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/outer/outer.html
new file mode 100644
index 0000000000..87ba71234a
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/outer/outer.html
@@ -0,0 +1,5 @@
+<template>
+    <x-inner>
+        <div slot="foo">yolo</div>
+    </x-inner>
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/outer/outer.js b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/outer/outer.js
new file mode 100644
index 0000000000..0ac1a1ba09
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/duplicate-slots-in-template/shadow/modules/x/outer/outer.js
@@ -0,0 +1,3 @@
+import { LightningElement} from 'lwc';
+
+export default class extends LightningElement {}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/error.txt b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/error.txt
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/expected.html b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/expected.html
new file mode 100644
index 0000000000..1130c9b17d
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/expected.html
@@ -0,0 +1,27 @@
+<x-outer>
+  <x-inner>
+    a
+    <!---->
+    <div>
+      x
+    </div>
+    <!---->
+    b
+    <!---->
+    <div>
+      y
+    </div>
+    <div>
+      y2
+    </div>
+    <div>
+      y3
+    </div>
+    <!---->
+    c
+    <!---->
+    default1default2default3default4default5default6
+    <!---->
+    d
+  </x-inner>
+</x-outer>
\ No newline at end of file
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/index.js b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/index.js
new file mode 100644
index 0000000000..17a015b5ea
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/index.js
@@ -0,0 +1,3 @@
+export const tagName = 'x-outer';
+export { default } from 'x/outer';
+export * from 'x/outer';
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/inner/inner.html b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/inner/inner.html
new file mode 100644
index 0000000000..cb2a076ac1
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/inner/inner.html
@@ -0,0 +1,9 @@
+<template lwc:render-mode="light">
+    a
+    <slot name="x"></slot>
+    b
+    <slot name="y"></slot>
+    c
+    <slot></slot>
+    d
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/inner/inner.js b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/inner/inner.js
new file mode 100644
index 0000000000..2104845f74
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/inner/inner.js
@@ -0,0 +1,5 @@
+import { LightningElement} from 'lwc';
+
+export default class extends LightningElement {
+  static renderMode = 'light'
+}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/outer/outer.html b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/outer/outer.html
new file mode 100644
index 0000000000..1736ecae2f
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/outer/outer.html
@@ -0,0 +1,15 @@
+<template lwc:render-mode="light">
+    <x-inner>
+        default1
+        <div slot="y">y</div>
+        default2
+        <div slot="x">x</div>
+        default3
+        <div slot="y">y2</div>
+        default4
+        <div slot="y">y3</div>
+        default5
+        <div slot="does-not-exist">does-not-exist</div>
+        default6
+    </x-inner>
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/outer/outer.js b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/outer/outer.js
new file mode 100644
index 0000000000..2104845f74
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/light/modules/x/outer/outer.js
@@ -0,0 +1,5 @@
+import { LightningElement} from 'lwc';
+
+export default class extends LightningElement {
+  static renderMode = 'light'
+}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/error.txt b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/error.txt
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/expected.html b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/expected.html
new file mode 100644
index 0000000000..9b3f40fb6d
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/expected.html
@@ -0,0 +1,39 @@
+<x-outer>
+  <template shadowrootmode="open">
+    <x-inner>
+      <template shadowrootmode="open">
+        a
+        <slot name="x">
+        </slot>
+        b
+        <slot name="y">
+        </slot>
+        c
+        <slot>
+        </slot>
+        d
+      </template>
+      default1
+      <div slot="y">
+        y
+      </div>
+      default2
+      <div slot="x">
+        x
+      </div>
+      default3
+      <div slot="y">
+        y2
+      </div>
+      default4
+      <div slot="y">
+        y3
+      </div>
+      default5
+      <div slot="does-not-exist">
+        does-not-exist
+      </div>
+      default6
+    </x-inner>
+  </template>
+</x-outer>
\ No newline at end of file
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/index.js b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/index.js
new file mode 100644
index 0000000000..17a015b5ea
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/index.js
@@ -0,0 +1,3 @@
+export const tagName = 'x-outer';
+export { default } from 'x/outer';
+export * from 'x/outer';
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/inner/inner.html b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/inner/inner.html
new file mode 100644
index 0000000000..fe27ea87d8
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/inner/inner.html
@@ -0,0 +1,9 @@
+<template>
+    a
+    <slot name="x"></slot>
+    b
+    <slot name="y"></slot>
+    c
+    <slot></slot>
+    d
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/inner/inner.js b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/inner/inner.js
new file mode 100644
index 0000000000..0ac1a1ba09
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/inner/inner.js
@@ -0,0 +1,3 @@
+import { LightningElement} from 'lwc';
+
+export default class extends LightningElement {}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/outer/outer.html b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/outer/outer.html
new file mode 100644
index 0000000000..0c2e25c0e9
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/outer/outer.html
@@ -0,0 +1,15 @@
+<template>
+    <x-inner>
+        default1
+        <div slot="y">y</div>
+        default2
+        <div slot="x">x</div>
+        default3
+        <div slot="y">y2</div>
+        default4
+        <div slot="y">y3</div>
+        default5
+        <div slot="does-not-exist">does-not-exist</div>
+        default6
+    </x-inner>
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/outer/outer.js b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/outer/outer.js
new file mode 100644
index 0000000000..0ac1a1ba09
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/reordered-duplicate-nonexistent-slots/shadow/modules/x/outer/outer.js
@@ -0,0 +1,3 @@
+import { LightningElement} from 'lwc';
+
+export default class extends LightningElement {}