If there is a callback URL delivered (which is not necessarily the notice's URI), such as a <link rel="alternate" ... /> then that could be used to retrieve the notice. But the hard part is to verify the author (and I've somewhat prepared the # resource lookup to be able to handle non-user lookups for this).

My idea is:
0. If we already know of the notice's URI, skip all of this and accept the repeat.
1. Find the profile of the supposed author of the repeated notice.
2. Contact the author's domain and do a WebFinger lookup with ?resource={$notice->uri}
3. Import the notice using info from WebFinger response and then apply the repeat on the imported notice.