Core: Fails the branch creation on an empty table when the branch exists

core/src/main/java/org/apache/iceberg/SnapshotManager.java

@@ -75,6 +75,8 @@ public ManageSnapshots createBranch(String name) {
       return createBranch(name, currentSnapshot.snapshotId());
     }
 
+    SnapshotRef existingRef = transaction.currentMetadata().ref(name);
+    Preconditions.checkArgument(existingRef == null, "Ref %s already exists", name);
     // Create an empty snapshot for the branch
     transaction.newFastAppend().toBranch(name).commit();
     return this;

core/src/test/java/org/apache/iceberg/TestSnapshotManager.java

@@ -262,6 +262,22 @@ public void testCreateBranchOnEmptyTable() {
     Assertions.assertThat(snapshot.removedDeleteFiles(table.io())).isEmpty();
   }
 
+  @Test
+  public void testCreateBranchOnEmptyTableFailsWhenRefAlreadyExists() {
+    table.manageSnapshots().createBranch("branch1").commit();
+
+    // Trying to create a branch with an existing name should fail
+    Assertions.assertThatThrownBy(() -> table.manageSnapshots().createBranch("branch1").commit())
+        .isInstanceOf(IllegalArgumentException.class)
+        .hasMessage("Ref branch1 already exists");
+
+    // Trying to create another branch within the same chain
+    Assertions.assertThatThrownBy(
+            () -> table.manageSnapshots().createBranch("branch2").createBranch("branch2").commit())
+        .isInstanceOf(IllegalArgumentException.class)
+        .hasMessage("Ref branch2 already exists");
+  }
+
   @Test
   public void testCreateBranchFailsWhenRefAlreadyExists() {
     table.newAppend().appendFile(FILE_A).commit();