Fix syntax_editor duplicated changed element

Example
---
```rust
let arg_list = make::arg_list([make::expr_literal("1").into(), make::expr_literal("2").into()]);
let mut editor = SyntaxEditor::new(arg_list.syntax().clone());
let target_expr = make::expr_literal("3").clone_for_update();

for arg in arg_list.args() {
    editor.replace(arg.syntax(), target_expr.syntax());
}

let edit = editor.finish();
let expect = expect![["(3, 3)"]];
expect.assert_eq(&edit.new_root.to_string());
```

**Before this PR**

```text
(, )3
```

**After this PR**

```text
(3, 3)
```

crates/ide-assists/src/handlers/convert_tuple_struct_to_named_struct.rs

@@ -1197,4 +1197,57 @@ fn foo() {
 "#,
         );
     }
+
+    #[test]
+    fn regression_issue_21020() {
+        check_assist(
+            convert_tuple_struct_to_named_struct,
+            r#"
+pub struct S$0(pub ());
+
+trait T {
+    fn id(&self) -> usize;
+}
+
+trait T2 {
+    fn foo(&self) -> usize;
+}
+
+impl T for S {
+    fn id(&self) -> usize {
+        self.0.len()
+    }
+}
+
+impl T2 for S {
+    fn foo(&self) -> usize {
+        self.0.len()
+    }
+}
+            "#,
+            r#"
+pub struct S { pub field1: () }
+
+trait T {
+    fn id(&self) -> usize;
+}
+
+trait T2 {
+    fn foo(&self) -> usize;
+}
+
+impl T for S {
+    fn id(&self) -> usize {
+        self.field1.len()
+    }
+}
+
+impl T2 for S {
+    fn foo(&self) -> usize {
+        self.field1.len()
+    }
+}
+            "#,
+        );
+    }
 }

crates/syntax/src/syntax_editor.rs

@@ -653,4 +653,40 @@ mod tests {
         let expect = expect![["fn it() {\n    \n}"]];
         expect.assert_eq(&edit.new_root.to_string());
     }
+
+    #[test]
+    fn test_more_times_replace_node_to_mutable() {
+        let arg_list =
+            make::arg_list([make::expr_literal("1").into(), make::expr_literal("2").into()]);
+
+        let mut editor = SyntaxEditor::new(arg_list.syntax().clone());
+        let target_expr = make::expr_literal("3").clone_for_update();
+
+        for arg in arg_list.args() {
+            editor.replace(arg.syntax(), target_expr.syntax());
+        }
+
+        let edit = editor.finish();
+
+        let expect = expect![["(3, 3)"]];
+        expect.assert_eq(&edit.new_root.to_string());
+    }
+
+    #[test]
+    fn test_more_times_insert_node_to_mutable() {
+        let arg_list =
+            make::arg_list([make::expr_literal("1").into(), make::expr_literal("2").into()]);
+
+        let mut editor = SyntaxEditor::new(arg_list.syntax().clone());
+        let target_expr = make::ext::expr_unit().clone_for_update();
+
+        for arg in arg_list.args() {
+            editor.insert(Position::before(arg.syntax()), target_expr.syntax());
+        }
+
+        let edit = editor.finish();
+
+        let expect = expect![["(()1, ()2)"]];
+        expect.assert_eq(&edit.new_root.to_string());
+    }
 }

crates/syntax/src/syntax_editor/edit_algo.rs

@@ -150,6 +150,15 @@ pub(super) fn apply_edits(editor: SyntaxEditor) -> SyntaxEdit {
     // Map change targets to the correct syntax nodes
     let tree_mutator = TreeMutator::new(&root);
     let mut changed_elements = vec![];
+    let mut changed_elements_set = rustc_hash::FxHashSet::default();
+    let mut deduplicate_node = |node_or_token: &mut SyntaxElement| {
+        let SyntaxElement::Node(node) = node_or_token else { return };
+        if changed_elements_set.contains(node) {
+            *node = node.clone_subtree().clone_for_update();
+        } else {
+            changed_elements_set.insert(node.clone());
+        }
+    };
 
     for index in independent_changes {
         match &mut changes[index as usize] {
@@ -180,6 +189,18 @@ pub(super) fn apply_edits(editor: SyntaxEditor) -> SyntaxEdit {
             }
         }
 
+        match &mut changes[index as usize] {
+            Change::Insert(_, element) | Change::Replace(_, Some(element)) => {
+                deduplicate_node(element);
+            }
+            Change::InsertAll(_, elements)
+            | Change::ReplaceWithMany(_, elements)
+            | Change::ReplaceAll(_, elements) => {
+                elements.iter_mut().for_each(&mut deduplicate_node);
+            }
+            Change::Replace(_, None) => (),
+        }
+
         // Collect changed elements
         match &changes[index as usize] {
             Change::Insert(_, element) => changed_elements.push(element.clone()),