diff --git a/.changeset/full-donuts-give.md b/.changeset/full-donuts-give.md
new file mode 100644
index 000000000..e31b8d69c
--- /dev/null
+++ b/.changeset/full-donuts-give.md
@@ -0,0 +1,5 @@
+---
+'mermaid': patch
+---
+
+fix: Log a warning when duplicate commit IDs are encountered in gitGraph to help identify and debug rendering issues caused by non-unique IDs.
diff --git a/packages/mermaid/src/diagrams/git/gitGraph.spec.ts b/packages/mermaid/src/diagrams/git/gitGraph.spec.ts
index fed21dd19..ee20a9aaf 100644
--- a/packages/mermaid/src/diagrams/git/gitGraph.spec.ts
+++ b/packages/mermaid/src/diagrams/git/gitGraph.spec.ts
@@ -1,4 +1,4 @@
-import { rejects } from 'assert';
+import { log } from '../../logger.js';
 import { db } from './gitGraphAst.js';
 import { parser } from './gitGraphParser.js';
 
@@ -1319,4 +1319,42 @@ describe('when parsing a gitGraph', function () {
       }
     });
   });
+  it('should log a warning when two commits have the same ID', async () => {
+    const str = `gitGraph
+     commit id:"initial commit"
+     commit id:"work on first release"
+     commit id:"design freeze from here"
+     branch v1-rc
+     checkout v1-rc
+     commit id:"bugfix 1"
+     commit id:"bigfix 2" tag:"v1.0.1"
+     branch FORK-v1.0-MDR
+     checkout FORK-v1.0-MDR
+     commit id:"working on MDR"
+     checkout v1-rc
+     commit id:"minor design changes for MDR" tag:"v1.0.2"
+     checkout FORK-v1.0-MDR
+     merge v1-rc
+     checkout main
+     commit id:"new feature for v1.1…"
+     checkout FORK-v1.0-MDR
+     commit id:"working on MDR"
+     commit id:"finishing MDR"
+     branch v1.0-MDR
+     checkout v1.0-MDR
+     commit id:"brush up release" tag:"v1.0.2-MDR"
+     checkout v1-rc
+     commit id:"bugfix without MDR"
+     checkout main
+     commit id:"work on v1.1"
+  `;
+
+    const logWarnSpy = vi.spyOn(log, 'warn').mockImplementation(() => undefined);
+
+    await parser.parse(str);
+
+    expect(logWarnSpy).toHaveBeenCalledWith('Commit ID working on MDR already exists');
+
+    logWarnSpy.mockRestore();
+  });
 });
diff --git a/packages/mermaid/src/diagrams/git/gitGraphAst.ts b/packages/mermaid/src/diagrams/git/gitGraphAst.ts
index 36595eb51..0dbc1ecb0 100644
--- a/packages/mermaid/src/diagrams/git/gitGraphAst.ts
+++ b/packages/mermaid/src/diagrams/git/gitGraphAst.ts
@@ -125,6 +125,9 @@ export const commit = function (commitDB: CommitDB) {
   };
   state.records.head = newCommit;
   log.info('main branch', config.mainBranchName);
+  if (state.records.commits.has(newCommit.id)) {
+    log.warn(`Commit ID ${newCommit.id} already exists`);
+  }
   state.records.commits.set(newCommit.id, newCommit);
   state.records.branches.set(state.records.currBranch, newCommit.id);
   log.debug('in pushCommit ' + newCommit.id);