diff --git a/Documentation/technical/reftable.md b/Documentation/technical/reftable.md
index 9e5c521fc..1236a7909 100644
--- a/Documentation/technical/reftable.md
+++ b/Documentation/technical/reftable.md
@@ -89,6 +89,10 @@ Reference names are an uninterpreted sequence of bytes that must pass
 
 [ref-fmt]: https://git-scm.com/docs/git-check-ref-format
 
+### Key unicity
+
+Each entry must have a unique key; repeated keys are disallowed.
+
 ### Network byte order
 
 All multi-byte, fixed width fields are in network byte order.
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java
index 5f5ab7267..53d13f1d6 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java
@@ -237,29 +237,6 @@ public class MergedReftableTest {
 		}
 	}
 
-	@Test
-	public void scanDuplicates() throws IOException {
-		List<Ref> delta1 = Arrays.asList(
-				ref("refs/heads/apple", 1),
-				ref("refs/heads/banana", 2));
-		List<Ref> delta2 = Arrays.asList(
-				ref("refs/heads/apple", 3),
-				ref("refs/heads/apple", 4));
-
-		MergedReftable mr = merge(write(delta1, 1000), write(delta2, 2000));
-		try (RefCursor rc = mr.allRefs()) {
-			assertTrue(rc.next());
-			assertEquals("refs/heads/apple", rc.getRef().getName());
-			assertEquals(id(3), rc.getRef().getObjectId());
-			assertEquals(2000, rc.getRef().getUpdateIndex());
-			assertTrue(rc.next());
-			assertEquals("refs/heads/banana", rc.getRef().getName());
-			assertEquals(id(2), rc.getRef().getObjectId());
-			assertEquals(1000, rc.getRef().getUpdateIndex());
-			assertFalse(rc.next());
-		}
-	}
-
 	@Test
 	public void scanIncludeDeletes() throws IOException {
 		List<Ref> delta1 = Arrays.asList(ref("refs/heads/next", 4));
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java
index 1c04733f6..45e6c7d12 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/ReftableTest.java
@@ -421,18 +421,20 @@ public class ReftableTest {
 	}
 
 	@Test
-	public void invalidRefWriteOrder() throws IOException {
+	public void invalidRefWriteOrderSortAndWrite() {
 		Ref master = ref(MASTER, 1);
-		Ref next = ref(NEXT, 2);
 		ReftableWriter writer = new ReftableWriter(new ByteArrayOutputStream())
 			.setMinUpdateIndex(1)
 			.setMaxUpdateIndex(1)
 			.begin();
 
-		writer.writeRef(next);
+		List<Ref> refs = new ArrayList<>();
+		refs.add(master);
+		refs.add(master);
+
 		IllegalArgumentException e  = assertThrows(
 			IllegalArgumentException.class,
-			() -> writer.writeRef(master));
+			() -> writer.sortAndWriteRefs(refs));
 		assertThat(e.getMessage(), containsString("records must be increasing"));
 	}
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java
index 0bb0a24d5..d06fee342 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java
@@ -230,6 +230,7 @@ public class ReftableWriter {
 
 	/**
 	 * Sort a collection of references and write them to the reftable.
+	 * The input refs may not have duplicate names.
 	 *
 	 * @param refsToPack
 	 *            references to sort and write.
@@ -243,10 +244,16 @@ public class ReftableWriter {
 				.map(r -> new RefEntry(r, maxUpdateIndex - minUpdateIndex))
 				.sorted(Entry::compare)
 				.iterator();
+		RefEntry last = null;
 		while (itr.hasNext()) {
 			RefEntry entry = itr.next();
+			if (last != null && Entry.compare(last, entry) == 0) {
+				throwIllegalEntry(last, entry);
+			}
+
 			long blockPos = refs.write(entry);
 			indexRef(entry.ref, blockPos);
+			last = entry;
 		}
 		return this;
 	}