cp: skip duplicate source check when --backup is given

src/uu/cp/src/cp.rs

@@ -1265,7 +1265,7 @@ pub fn copy(sources: &[PathBuf], target: &Path, options: &Options) -> CopyResult
 
     for source in sources {
         let normalized_source = normalize_path(source);
-        if seen_sources.contains(&normalized_source) {
+        if options.backup == BackupMode::NoBackup && seen_sources.contains(&normalized_source) {
             show_warning!("source file {} specified more than once", source.quote());
         } else {
             let dest = construct_dest_path(source, target, target_type, options)

tests/by-util/test_cp.rs

@@ -134,6 +134,33 @@ fn test_cp_duplicate_files_normalized_path() {
     assert_eq!(at.read(TEST_COPY_TO_FOLDER_FILE), "Hello, World!\n");
 }
 
+#[test]
+fn test_cp_duplicate_files_with_plain_backup() {
+    let (_, mut ucmd) = at_and_ucmd!();
+    ucmd.arg(TEST_HELLO_WORLD_SOURCE)
+        .arg(TEST_HELLO_WORLD_SOURCE)
+        .arg(TEST_COPY_TO_FOLDER)
+        .arg("--backup")
+        .fails()
+        // cp would skip duplicate src check and fail when it tries to overwrite the "just-created" file.
+        .stderr_contains(
+            "will not overwrite just-created 'hello_dir/hello_world.txt' with 'hello_world.txt",
+        );
+}
+
+#[test]
+fn test_cp_duplicate_files_with_numbered_backup() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    // cp would skip duplicate src check and succeeds
+    ucmd.arg(TEST_HELLO_WORLD_SOURCE)
+        .arg(TEST_HELLO_WORLD_SOURCE)
+        .arg(TEST_COPY_TO_FOLDER)
+        .arg("--backup=numbered")
+        .succeeds();
+    at.file_exists(TEST_COPY_TO_FOLDER_FILE);
+    at.file_exists(format!("{TEST_COPY_TO_FOLDER}.~1~"));
+}
+
 #[test]
 fn test_cp_same_file() {
     let (at, mut ucmd) = at_and_ucmd!();