fix: handle completion when already signed (#3159)

Previously attempting to complete a document which is already completed
you'd get a generic error toast. Now when completing a document that you
have already completed you are redirected to the completed page.

Handles cases where two mutations managed to fire racing eachother.

packages/lib/errors/app-error.ts

@@ -36,6 +36,13 @@ export enum AppErrorCode {
    */
   ENVELOPE_TSP_LOCKED = 'ENVELOPE_TSP_LOCKED',
 
+  /**
+   * A completion request was made for a recipient that has already signed.
+   * Thrown for retried, stale or concurrent duplicate submissions so callers
+   * can resolve them idempotently instead of surfacing an error.
+   */
+  RECIPIENT_ALREADY_SIGNED = 'RECIPIENT_ALREADY_SIGNED',
+
   /**
    * A signer recipient does not have a signature field assigned. Thrown when
    * distributing an envelope or using a direct template where at least one