diff --git a/src/clock/index.js b/src/clock/index.js
index 9eae784a..30a41774 100644
--- a/src/clock/index.js
+++ b/src/clock/index.js
@@ -122,6 +122,7 @@ const contains = async (events, a, b) => {
   if (a.toString() === b.toString()) return true
   const [{ value: aevent }, { value: bevent }] = await Promise.all([events.get(a), events.get(b)])
   const links = [...aevent.parents]
+  const seen = new Set()
   while (links.length) {
     const link = links.shift()
     if (!link) break
@@ -129,6 +130,8 @@ const contains = async (events, a, b) => {
     // if any of b's parents are this link, then b cannot exist in any of the
     // tree below, since that would create a cycle.
     if (bevent.parents.some(p => link.toString() === p.toString())) continue
+    if (seen.has(link.toString())) continue
+    seen.add(link.toString())
     const { value: event } = await events.get(link)
     links.push(...event.parents)
   }
diff --git a/test/clock.test.js b/test/clock.test.js
index 52b22d1a..b819e04d 100644
--- a/test/clock.test.js
+++ b/test/clock.test.js
@@ -146,6 +146,12 @@ describe('clock', () => {
 
   it('add an old event', async () => {
     const blocks = new Blockstore()
+    const blockGet = blocks.get.bind(blocks)
+    let count = 0
+    blocks.get = async cid => {
+      count++
+      return blockGet(cid)
+    }
     const root = await EventBlock.create(await randomEventData())
     await blocks.put(root.cid, root.bytes)
 
@@ -184,7 +190,9 @@ describe('clock', () => {
     // now very old one
     const event6 = await EventBlock.create(await randomEventData(), parents0)
     await blocks.put(event6.cid, event6.bytes)
+    const before = count
     head = await advance(blocks, head, event6.cid)
+    assert.equal(count - before, 10)
 
     for await (const line of vis(blocks, head)) console.log(line)
     assert.equal(head.length, 2)
@@ -212,4 +220,79 @@ describe('clock', () => {
     assert.equal(head.length, 1)
     assert.equal(head[0].toString(), event1.cid.toString())
   })
+
+
+  /*
+  ```mermaid
+  flowchart TB
+      event3 --> event1
+      event3 --> event2
+      event2 --> event0
+      event1 --> event0
+      event0 --> genesis
+      event4 --> genesis
+  ```
+
+  All we want to do is prove that `event0` and `genesis` are not fetched
+  multiple times, since there are multiple paths to it in the tree.
+
+  We arrive at 8, because when we advance with `head: [event3], event: event4`
+  we firstly check if the head is in the event:
+
+  * get event4 (1)
+  * get event3 (2)
+  * get genesis (3)
+
+  Then we check if the event is in the head, with de-duping:
+
+  * get event3 (4)
+  * get event4 (5)
+  * then get each of the nodes back to parent(s) of `event4` (`genesis`):
+      * event1 (6)
+      * event2 (7)
+      * event0 (8)
+      * (we don't fetch genesis due to existing cycle detection)
+
+  Without deduping, we expect 9 node fetches, since we traverse across `event0`
+  again, since it is linked to by 2 nodes. 
+   */
+  it('contains only traverses history once', async () => {
+    const blocks = new Blockstore()
+    const genesis = await EventBlock.create(await randomEventData())
+    await blocks.put(genesis.cid, genesis.bytes)
+    /** @type {API.EventLink<any>[]} */
+    let head = [genesis.cid]
+    const blockGet = blocks.get.bind(blocks)
+    let count = 0
+    blocks.get = async cid => {
+      count++
+      return blockGet(cid)
+    }
+
+    const event0 = await EventBlock.create(await randomEventData(), [genesis.cid])
+    await blocks.put(event0.cid, event0.bytes)
+    head = await advance(blocks, head, event0.cid)
+
+    const event1 = await EventBlock.create(await randomEventData(), [event0.cid])
+    await blocks.put(event1.cid, event1.bytes)
+    head = await advance(blocks, head, event1.cid)
+
+    const event2 = await EventBlock.create(await randomEventData(), [event0.cid])
+    await blocks.put(event2.cid, event2.bytes)
+    head = await advance(blocks, head, event2.cid)
+
+    const event3 = await EventBlock.create(await randomEventData(), [event1.cid, event2.cid])
+    await blocks.put(event3.cid, event3.bytes)
+    head = await advance(blocks, head, event3.cid)
+
+    const before = count
+    const event4 = await EventBlock.create(await randomEventData(), [genesis.cid])
+    await blocks.put(event4.cid, event4.bytes)
+    head = await advance(blocks, head, event4.cid)
+
+    assert.equal(head.length, 2)
+    assert.equal(head[1].toString(), event4.cid.toString())
+    assert.equal(head[0].toString(), event3.cid.toString())
+    assert.equal(count - before, 8, 'The number of traversals should be 8 with optimization')
+  })
 })