diff --git a/crates/engine/src/cmd/modify2/curve_edits.rs b/crates/engine/src/cmd/modify2/curve_edits.rs
index 31ef757..b27f9a5 100644
--- a/crates/engine/src/cmd/modify2/curve_edits.rs
+++ b/crates/engine/src/cmd/modify2/curve_edits.rs
@@ -228,6 +228,9 @@ fn ends(sp: &Spline) -> (Vec2, Vec2) {
 /// larger arc or the full ellipse (gaps between them close). Otherwise the objects joined end
 /// to end with the first one become one spline. The first joined object keeps its handle.
 pub(crate) fn join_curves(s: &mut Session, hs: &[Handle]) -> Result<Option<Handle>> {
+    // A handle named twice is one object: a repeat would join it with itself or remove the result.
+    let mut seen = std::collections::HashSet::new();
+    let hs: Vec<Handle> = hs.iter().copied().filter(|h| seen.insert(*h)).collect();
     let d = s.doc()?;
     let ents: Vec<Entity> = hs.iter().filter_map(|h| d.entity(*h).map(|e| (**e).clone())).collect();
     if !ents.iter().any(|e| matches!(e.kind, EntityKind::Ellipse(_) | EntityKind::Spline(_))) {
diff --git a/crates/engine/tests/modify_curves.rs b/crates/engine/tests/modify_curves.rs
index b8a869b..cea710d 100644
--- a/crates/engine/tests/modify_curves.rs
+++ b/crates/engine/tests/modify_curves.rs
@@ -262,6 +262,26 @@ fn join_merges_elliptical_arcs_on_one_ellipse() {
     assert_eq!(count(&s), 1);
 }
 
+#[test]
+fn join_names_an_elliptical_arc_twice_without_losing_it() {
+    // The same arc twice is one object: nothing to join, and the arc stays.
+    let mut s = Session::new();
+    let a = arc_on_ellipse(&mut s, 0.0, 90.0);
+    let before = ellipse(&s, &a);
+    assert!(s.execute("join", &json!({ "handles": [a, a] })).is_err());
+    assert_eq!(count(&s), 1);
+    assert_eq!(ellipse(&s, &a), before);
+    // Repeating the source next to a second arc still merges the two into the source.
+    let mut s = Session::new();
+    let a = arc_on_ellipse(&mut s, 0.0, 90.0);
+    let b = arc_on_ellipse(&mut s, 90.0, 180.0);
+    let r = s.execute("join", &json!({ "handles": [a, a, b] })).unwrap();
+    assert_eq!(h(&r), a);
+    let m = ellipse(&s, &a);
+    assert!(m.start.abs() < 1e-9 && (m.end - PI).abs() < 1e-9, "{m:?}");
+    assert_eq!(count(&s), 1);
+}
+
 #[test]
 fn join_makes_one_spline_from_lines_arcs_polylines_and_splines() {
     let mut s = Session::new();