diff --git a/Sources/AutomergeRepo/Errors.swift b/Sources/AutomergeRepo/Errors.swift
index e1e2afc..3faefe3 100644
--- a/Sources/AutomergeRepo/Errors.swift
+++ b/Sources/AutomergeRepo/Errors.swift
@@ -93,4 +93,16 @@ public enum Errors: Sendable {
             self.msg = msg
         }
     }
+    
+    /// The ID of the document already exists within the repository
+    public struct DuplicateID: Sendable, LocalizedError {
+        public var id: DocumentId
+        public var errorDescription: String? {
+            "The ID of the document \(id) already exists within the repository."
+        }
+
+        public init(id: DocumentId) {
+            self.id = id
+        }
+    }
 }
diff --git a/Sources/AutomergeRepo/Repo.swift b/Sources/AutomergeRepo/Repo.swift
index b57db52..bf554c8 100644
--- a/Sources/AutomergeRepo/Repo.swift
+++ b/Sources/AutomergeRepo/Repo.swift
@@ -469,6 +469,9 @@ public final class Repo {
     /// - Returns: The Automerge document.
     /// - Parameter id: The Id of the Automerge document.
     public func create(id: DocumentId) async throws -> DocHandle {
+        if let existing = handles[id] {
+            throw Errors.DuplicateID(id: id)
+        }
         let handle = InternalDocHandle(id: id, isNew: true, initialValue: Document())
         handles[handle.id] = handle
         docHandlePublisher.send(handle.snapshot())
@@ -480,6 +483,9 @@ public final class Repo {
     /// - Parameter doc: The Automerge document to use for the new, shared document
     /// - Returns: The Automerge document.
     public func create(doc: Document, id: DocumentId? = nil) async throws -> DocHandle {
+        if let providedId = id, let existing = handles[providedId] {
+            throw Errors.DuplicateID(id: providedId)
+        }
         let creationId = id ?? DocumentId()
         let handle = InternalDocHandle(id: creationId, isNew: true, initialValue: doc)
         handles[handle.id] = handle
@@ -492,6 +498,9 @@ public final class Repo {
     /// - Parameter data: The data to load as an Automerge document for the new, shared document.
     /// - Returns: The Automerge document.
     public func create(data: Data, id: DocumentId? = nil) async throws -> DocHandle {
+        if let providedId = id, let existing = handles[providedId] {
+            throw Errors.DuplicateID(id: providedId)
+        }
         let creationId = id ?? DocumentId()
         let handle = try InternalDocHandle(id: creationId, isNew: true, initialValue: Document(data))
         handles[handle.id] = handle