copies: keep renames detected against multiple merge parents

CHANGELOG.md

@@ -25,6 +25,11 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   aliases that are simply repeated. For example, with the alias `jj = []`, the
   command `jj jj jj log` will resolve to `jj log` as expected.
 
+* `jj diff` and `jj status` no longer omit a rename or copy in a merge commit
+  when the renamed source is present in more than one parent. The identical copy
+  record reported for each parent was mistaken for a conflict and discarded.
+  [#9752](https://github.com/jj-vcs/jj/issues/9752)
+
 ## [0.43.0] - 2026-07-01
 
 ### Release highlights

cli/tests/test_diff_command.rs

@@ -4089,3 +4089,32 @@ fn test_diff_revisions() {
     [EOF]
     ");
 }
+
+#[test]
+fn test_diff_rename_in_merge_commit() {
+    let test_env = TestEnvironment::default();
+    test_env.run_jj_in(".", ["git", "init", "repo"]).success();
+    let work_dir = test_env.work_dir("repo");
+
+    // A rename in a merge commit is detected once per parent. When the renamed
+    // source is present in more than one parent, the same copy record is reported
+    // for each, and those duplicates must not cancel each other out and drop the
+    // rename. https://github.com/jj-vcs/jj/issues/9752
+    work_dir.write_file("a.txt", "aaa\n");
+    work_dir.run_jj(["describe", "-m", "base"]).success();
+    work_dir.run_jj(["new", "-m", "first branch"]).success();
+    work_dir
+        .run_jj(["new", "@-", "-m", "second branch"])
+        .success();
+    // Merge both children of "base" (@-+); each holds a.txt, so the rename is
+    // detected against both parents.
+    work_dir.run_jj(["new", "@-+", "-m", "merge"]).success();
+    work_dir.remove_file("a.txt");
+    work_dir.write_file("b.txt", "aaa\n");
+
+    let output = work_dir.run_jj(["diff", "-s"]);
+    insta::assert_snapshot!(output, @"
+    R {a.txt => b.txt}
+    [EOF]
+    ");
+}

cli/tests/test_diffedit_command.rs

@@ -710,7 +710,7 @@ fn test_diffedit_merge() -> TestResult {
     let output = work_dir.run_jj(["diff", "-r", "@-", "-s"]);
     insta::assert_snapshot!(output, @"
     M file1
-    A file3
+    C {file1 => file3}
     [EOF]
     ");
 

lib/src/copies.rs

@@ -65,6 +65,18 @@ impl CopyRecords {
     /// conflicts is discarded and treated as not having an origin.
     pub fn add_records(&mut self, copy_records: impl IntoIterator<Item = CopyRecord>) {
         for r in copy_records {
+            // The same copy or rename is reported once per parent when diffing a
+            // merge commit. Identical (source, target) pairs describe the same
+            // operation, so skip the duplicate instead of marking both maps as
+            // conflicting, which would otherwise drop the copy/rename entirely.
+            let is_duplicate = self
+                .targets
+                .get(&r.target)
+                .and_then(|&i| self.records.get(i))
+                .is_some_and(|existing| existing.source == r.source);
+            if is_duplicate {
+                continue;
+            }
             self.sources
                 .entry(r.source.clone())
                 // TODO: handle conflicts instead of ignoring both sides.