diff --git a/org.eclipse.jgit.test/exttst/org/eclipse/jgit/internal/storage/file/CGitLockFileTest.java b/org.eclipse.jgit.test/exttst/org/eclipse/jgit/internal/storage/file/CGitLockFileTest.java
new file mode 100644
index 0000000..4c19475
--- /dev/null
+++ b/org.eclipse.jgit.test/exttst/org/eclipse/jgit/internal/storage/file/CGitLockFileTest.java
@@ -0,0 +1,57 @@
+/*
+ * Copyright (C) 2021 SAP SE and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.internal.storage.file;
+
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+import org.eclipse.jgit.api.Git;
+import org.eclipse.jgit.junit.RepositoryTestCase;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.util.FS;
+import org.eclipse.jgit.util.FS.ExecutionResult;
+import org.junit.Test;
+
+/**
+ * Unit tests of {@link LockFile} testing interoperability with C git
+ */
+public class CGitLockFileTest extends RepositoryTestCase {
+
+	@Test
+	public void testLockedTwiceFails() throws Exception {
+		try (Git git = new Git(db)) {
+			writeTrashFile("file.txt", "content");
+			git.add().addFilepattern("file.txt").call();
+			RevCommit commit1 = git.commit().setMessage("create file").call();
+
+			assertNotNull(commit1);
+			writeTrashFile("file.txt", "content2");
+			git.add().addFilepattern("file.txt").call();
+			assertNotNull(git.commit().setMessage("edit file").call());
+
+			LockFile lf = new LockFile(db.getIndexFile());
+			assertTrue(lf.lock());
+			try {
+				String[] command = new String[] { "git", "checkout",
+						commit1.name() };
+				ProcessBuilder pb = new ProcessBuilder(command);
+				pb.directory(db.getWorkTree());
+				ExecutionResult result = FS.DETECTED.execute(pb, null);
+				assertNotEquals(0, result.getRc());
+				String err = result.getStderr().toString().split("\\R")[0];
+				assertTrue(err.matches(
+						"fatal: Unable to create .*/\\.git/index\\.lock': File exists\\."));
+			} finally {
+				lf.unlock();
+			}
+		}
+	}
+}