@@ -1643,7 +1643,8 @@ static int set_head(const struct ref *remote_refs, struct remote *remote)
 
 struct ref_rejection_data {
 	int *retcode;
-	int conflict_msg_shown;
+	bool conflict_msg_shown;
+	bool case_sensitive_msg_shown;
 	const char *remote_name;
 };
 

@@ -1657,11 +1658,25 @@ static void ref_transaction_rejection_handler(const char *refname,
 {
 	struct ref_rejection_data *data = cb_data;
 
-	if (err == REF_TRANSACTION_ERROR_NAME_CONFLICT && !data->conflict_msg_shown) {
+	if (err == REF_TRANSACTION_ERROR_CREATE_EXISTS && ignore_case &&
+	    !data->case_sensitive_msg_shown) {
+		error(_("You're on a case-insensitive filesystem, and the remote you are\n"
+			"trying to fetch from has references that only differ in casing. It\n"
+			"is impossible to store such references with the 'files' backend. You\n"
+			"can either accept this as-is, in which case you won't be able to\n"
+			"store all remote references on disk. Or you can alternatively\n"
+			"migrate your repository to use the 'reftable' backend with the\n"
+			"following command:\n\n    git refs migrate --ref-format=reftable\n\n"
+			"Please keep in mind that not all implementations of Git support this\n"
+			"new format yet. So if you use tools other than Git to access this\n"
+			"repository it may not be an option to migrate to reftables.\n"));
+		data->case_sensitive_msg_shown = true;
+	} else if (err == REF_TRANSACTION_ERROR_NAME_CONFLICT &&
+		   !data->conflict_msg_shown) {
 		error(_("some local refs could not be updated; try running\n"
 			" 'git remote prune %s' to remove any old, conflicting "
 			"branches"), data->remote_name);
-		data->conflict_msg_shown = 1;
+		data->conflict_msg_shown = true;
 	} else {
 		const char *reason = ref_transaction_error_msg(err);
 

@@ -776,6 +776,8 @@ static enum ref_transaction_error lock_raw_ref(struct files_ref_store *refs,
 			goto retry;
 		} else {
 			unable_to_lock_message(ref_file.buf, myerr, err);
+			if (myerr == EEXIST)
+				ret = REF_TRANSACTION_ERROR_CREATE_EXISTS;
 			goto error_return;
 		}
 	}

@@ -2294,6 +2294,30 @@ do
 		)
 	'
 
+	test_expect_success CASE_INSENSITIVE_FS,REFFILES "stdin $type batch-updates existing reference" '
+		git init repo &&
+		test_when_finished "rm -fr repo" &&
+		(
+			cd repo &&
+			test_commit one &&
+			old_head=$(git rev-parse HEAD) &&
+			test_commit two &&
+			head=$(git rev-parse HEAD) &&
+
+			format_command $type "create refs/heads/foo" "$head" >stdin &&
+			format_command $type "create refs/heads/ref" "$old_head" >>stdin &&
+			format_command $type "create refs/heads/Foo" "$old_head" >>stdin &&
+			git update-ref $type --stdin --batch-updates <stdin >stdout &&
+
+			echo $head >expect &&
+			git rev-parse refs/heads/foo >actual &&
+			echo $old_head >expect &&
+			git rev-parse refs/heads/ref >actual &&
+			test_cmp expect actual &&
+			test_grep -q "reference already exists" stdout
+		)
+	'
+
 	test_expect_success "stdin $type batch-updates delete incorrect symbolic ref" '
 		git init repo &&
 		test_when_finished "rm -fr repo" &&

@@ -47,7 +47,13 @@ test_expect_success "clone and setup child repos" '
 		git config set branch.main.merge refs/heads/one
 	) &&
 	git clone . bundle &&
-	git clone . seven
+	git clone . seven &&
+	git clone --ref-format=reftable . case_sensitive &&
+	(
+		cd case_sensitive &&
+		git branch branch1 &&
+		git branch bRanch1
+	)
 '
 
 test_expect_success "fetch test" '

@@ -1526,6 +1532,20 @@ test_expect_success SYMLINKS 'clone does not get confused by a D/F conflict' '
 	test_path_is_missing whoops
 '
 
+test_expect_success CASE_INSENSITIVE_FS,REFFILES 'existing references in a case insensitive filesystem' '
+	test_when_finished rm -rf case_insensitive &&
+	(
+		git init --bare case_insensitive &&
+		cd case_insensitive &&
+		git remote add origin -- ../case_sensitive &&
+		test_must_fail git fetch -f origin "refs/heads/*:refs/heads/*" 2>err &&
+		test_grep "You${SQ}re on a case-insensitive filesystem" err &&
+		git rev-parse refs/heads/main >expect &&
+		git rev-parse refs/heads/branch1 >actual &&
+		test_cmp expect actual
+	)
+'
+
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd