diff --git a/commit-reach.c b/commit-reach.c
index d3a9b3ed6fe561..82614d24096304 100644
--- a/commit-reach.c
+++ b/commit-reach.c
@@ -1125,7 +1125,6 @@ void ahead_behind(struct repository *r,
 
 struct commit_and_index {
 	struct commit *commit;
-	unsigned int index;
 	timestamp_t generation;
 };
 
@@ -1165,7 +1164,6 @@ void tips_reachable_from_bases(struct repository *r,
 
 	for (size_t i = 0; i < tips_nr; i++) {
 		commits[i].commit = tips[i];
-		commits[i].index = i;
 		commits[i].generation = commit_graph_generation(tips[i]);
 	}
 
@@ -1173,6 +1171,9 @@ void tips_reachable_from_bases(struct repository *r,
 	QSORT(commits, tips_nr, compare_commit_and_index_by_generation);
 	min_generation = commits[0].generation;
 
+	for (size_t i = 0; i < tips_nr; i++)
+		commits[i].commit->object.flags |= RESULT;
+
 	while (bases) {
 		repo_parse_commit(r, bases->item);
 		commit_list_insert(bases->item, &stack);
@@ -1183,20 +1184,16 @@ void tips_reachable_from_bases(struct repository *r,
 		int explored_all_parents = 1;
 		struct commit_list *p;
 		struct commit *c = stack->item;
-		timestamp_t c_gen = commit_graph_generation(c);
 
 		/* Does it match any of our tips? */
-		for (size_t j = min_generation_index; j < tips_nr; j++) {
-			if (c_gen < commits[j].generation)
-				break;
-
-			if (commits[j].commit == c) {
-				tips[commits[j].index]->object.flags |= mark;
+		{
+			if (c->object.flags & RESULT) {
+				c->object.flags |= mark;
 
-				if (j == min_generation_index) {
-					unsigned int k = j + 1;
+				if (commits[min_generation_index].commit->object.flags & mark) {
+					unsigned int k = min_generation_index + 1;
 					while (k < tips_nr &&
-					       (tips[commits[k].index]->object.flags & mark))
+					       (commits[k].commit->object.flags & mark))
 						k++;
 
 					/* Terminate early if all found. */
@@ -1232,6 +1229,8 @@ void tips_reachable_from_bases(struct repository *r,
 	}
 
 done:
+	for (size_t i = 0; i < tips_nr; i++)
+		commits[i].commit->object.flags &= ~RESULT;
 	free(commits);
 	repo_clear_commit_marks(r, SEEN);
 	commit_list_free(stack);
diff --git a/t/t6600-test-reach.sh b/t/t6600-test-reach.sh
index dc0421ed2f3726..9486002866fb13 100755
--- a/t/t6600-test-reach.sh
+++ b/t/t6600-test-reach.sh
@@ -612,6 +612,51 @@ test_expect_success 'for-each-ref merged:none' '
 		--format="%(refname)" --stdin
 '
 
+test_expect_success 'for-each-ref merged:duplicate, all reachable' '
+	git branch dup-a commit-3-3 &&
+	git branch dup-b commit-3-3 &&
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/dup-a
+	refs/heads/dup-b
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/dup-a
+	refs/heads/dup-b
+	EOF
+	run_all_modes git for-each-ref --merged=commit-5-5 \
+		--format="%(refname)" --stdin
+'
+
+test_expect_success 'for-each-ref merged:duplicate, none reachable' '
+	cat >input <<-\EOF &&
+	refs/heads/dup-a
+	refs/heads/dup-b
+	refs/heads/commit-9-9
+	EOF
+	>expect &&
+	run_all_modes git for-each-ref --merged=commit-2-2 \
+		--format="%(refname)" --stdin
+'
+
+test_expect_success 'for-each-ref merged:duplicate at min generation' '
+	git branch dup-c commit-1-1 &&
+	git branch dup-d commit-1-1 &&
+	cat >input <<-\EOF &&
+	refs/heads/dup-c
+	refs/heads/dup-d
+	refs/heads/commit-5-5
+	EOF
+	cat >expect <<-\EOF &&
+	refs/heads/commit-5-5
+	refs/heads/dup-c
+	refs/heads/dup-d
+	EOF
+	run_all_modes git for-each-ref --merged=commit-5-5 \
+		--format="%(refname)" --stdin
+'
+
 # For get_branch_base_for_tip, we only care about
 # first-parent history. Here is the test graph with
 # second parents removed: