diff --git a/scripts/report-unmerged-branches.mjs b/scripts/report-unmerged-branches.mjs
index 241d08f..aac4057 100644
--- a/scripts/report-unmerged-branches.mjs
+++ b/scripts/report-unmerged-branches.mjs
@@ -42,6 +42,15 @@ export function inspectRepository(repository, { fetch = false } = {}) {
     const output = git(repository.path, 'for-each-ref', '--format=%(refname)%00%(objectname)%00%(symref)', `refs/remotes/${remote}/`);
     const cache = new Map();
     const knownEquivalent = new Set();
+    const mergeBases = new Map();
+    const isolatedEquivalence = new Map();
+    const mergeBase = sha => {
+      if (!mergeBases.has(sha)) {
+        try { mergeBases.set(sha, git(repository.path, 'merge-base', result.targetSha, sha)); }
+        catch (error) { if (error.status === 1 && !String(error.stderr || '').trim()) mergeBases.set(sha, null); else throw error; }
+      }
+      return mergeBases.get(sha);
+    };
     for (const line of output.split('\n').filter(Boolean)) {
       const [ref, sha, symbolicRef] = line.split('\0');
       if (symbolicRef || ref === targetRef) continue;
@@ -49,20 +58,24 @@ export function inspectRepository(repository, { fetch = false } = {}) {
       try {
         let comparison = cache.get(sha);
         if (!comparison) {
-          const range = `${targetRef}..${ref}`;
+          // Pin the inventory SHAs so another fetch cannot change this comparison midway.
+          const range = `${result.targetSha}..${sha}`;
           const ahead = Number(git(repository.path, 'rev-list', '--count', range));
           comparison = { ahead, unmergedCommits: [], equivalentCommits: [], mergeCommits: [] };
           if (ahead > 0) {
-            const cherry = git(repository.path, 'cherry', targetRef, ref);
+            const cherry = git(repository.path, 'cherry', result.targetSha, sha);
             const equivalence = new Map(cherry.split('\n').filter(Boolean).map(line => [line.slice(2), line[0]]));
             comparison.mergeCommits = commits(repository.path, range, '--merges');
             for (const commit of commits(repository.path, range, '--no-merges')) {
               if (equivalence.get(commit.sha) === '-') knownEquivalent.add(commit.sha);
               // If the branch merged the target, Git's merge base can hide older
               // upstream patches. Compare the individual incoming commit too.
-              if (comparison.mergeCommits.length && !knownEquivalent.has(commit.sha)) {
-                const isolated = git(repository.path, 'cherry', targetRef, commit.sha);
-                for (const line of isolated.split('\n')) if (line.startsWith('- ')) knownEquivalent.add(line.slice(2));
+              if (comparison.mergeCommits.length && !knownEquivalent.has(commit.sha) && mergeBase(sha) !== mergeBase(commit.sha)) {
+                if (!isolatedEquivalence.has(commit.sha)) {
+                  const isolated = git(repository.path, 'cherry', result.targetSha, commit.sha);
+                  isolatedEquivalence.set(commit.sha, isolated.split('\n').includes('- ' + commit.sha));
+                }
+                if (isolatedEquivalence.get(commit.sha)) knownEquivalent.add(commit.sha);
               }
               // A changed SHA alone does not mean the change is absent from the target branch.
               const list = knownEquivalent.has(commit.sha) ? comparison.equivalentCommits : comparison.unmergedCommits;