log warning when duplicate commit IDs are encountered

cypress/integration/rendering/gitGraph.spec.js

@@ -1569,4 +1569,37 @@ gitGraph TB:
       {}
     );
   });
+  it('77:should render gitGraph with duplicate commit IDs and log warnings', () => {
+    imgSnapshotTest(
+      `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"`,
+      {}
+    );
+    cy.log('Warning: Commit ID "working on MDR" already exists');
+  });
 });

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);