fix(tree): handle redundant sequencing of local commit (#25544)

## Description

Fixes a bug in the scenario where a commit from a child shared branch is
concurrently merged into the main shared branch by different peers. The
peer whose merged commits are not sequenced first had a mistaken
expectation that it should have a commit on its main local branch for
each commit that is sequenced by the service if that commit came from
this peer. This is no longer a valid expectation.

## Breaking Changes

None

Author: yann-achard-MS

packages/dds/tree/src/shared-tree-core/editManager.ts

@@ -750,8 +750,8 @@ class SharedBranch<TEditor extends ChangeFamilyEditor, TChangeset> {
 		// Local changes, i.e., changes from this client are applied by fast forwarding the local branch commit onto
 		// the trunk.
 		if (areLocalCommits) {
-			for (const _ of newCommits) {
-				this.sequenceLocalCommit(nextSequenceId, sessionId, onSequenceLocalCommit);
+			for (const { revision } of newCommits) {
+				this.sequenceLocalCommit(revision, nextSequenceId, sessionId, onSequenceLocalCommit);
 				nextSequenceId = getNextSequenceId(nextSequenceId);
 			}
 			return;
@@ -900,10 +900,17 @@ class SharedBranch<TEditor extends ChangeFamilyEditor, TChangeset> {
 	 * Avoiding the overhead of the rebase process, even when it's a no-op, has real measured performance benefits and is worth the added complexity here.
 	 */
 	private sequenceLocalCommit(
+		revision: RevisionTag,
 		sequenceId: SequenceId,
 		sessionId: SessionId,
 		onSequenceLocalCommit: OnSequenceCommit<TChangeset>,
-	): GraphCommit<TChangeset> {
+	): void {
+		if (this.commitMetadata.has(revision)) {
+			// This can happen if the commit came from a shared branch and was concurrently merged by another client.
+			// In this case, the newly sequenced commit is redundant and should therefore be ignored.
+			return;
+		}
+
 		// First, push the local commit to the trunk.
 		// We are mutating our `localCommits` cache here,but there is no need to actually change the `localBranch` itself because it will simply catch up later if/when it next rebases.
 		const firstLocalCommit = this.localCommits.shift();
@@ -915,7 +922,6 @@ class SharedBranch<TEditor extends ChangeFamilyEditor, TChangeset> {
 		const prevSequenceId = this.getCommitSequenceId(this.trunk.getHead().revision);
 		this.pushGraphCommitToTrunk(sequenceId, firstLocalCommit, sessionId);
 		onSequenceLocalCommit(firstLocalCommit, sequenceId, prevSequenceId);
-		return firstLocalCommit;
 	}
 
 	/**