feat: add props.onDuplicate (#9117)

* feat: add `props.onDuplicate`

* docs

* clarify docs

* fix docs

packages/excalidraw/types.ts

@@ -512,6 +512,22 @@ export interface ExcalidrawProps {
     data: ClipboardData,
     event: ClipboardEvent | null,
   ) => Promise<boolean> | boolean;
+  /**
+   * Called when element(s) are duplicated so you can listen or modify as
+   * needed.
+   *
+   * Called when duplicating via mouse-drag, keyboard, paste, library insert
+   * etc.
+   *
+   * Returned elements will be used in place of the next elements
+   * (you should return all elements, including deleted, and not mutate
+   * the element if changes are made)
+   */
+  onDuplicate?: (
+    nextElements: readonly ExcalidrawElement[],
+    /** excludes the duplicated elements */
+    prevElements: readonly ExcalidrawElement[],
+  ) => ExcalidrawElement[] | void;
   renderTopRightUI?: (
     isMobile: boolean,
     appState: UIAppState,