Deliveries to followers carry a digest of them, per server

Owner decision of 2026-10-06 (FEP-8fcf). A persona's delivery addressed to its followers carries a signed
Collection-Synchronization header naming its followers, its roll-call (…/groupies/roll-call) and the digest of its
accepted followers on the receiving server only. The roll-call answers a signed request with the persona's followers
on the signer's server and nobody else's.

Mastodon gives every Undo{Follow} it sends after reading a roll-call the same id (…#follows//undo), so a second one
looked like a copy: an Undo of a Follow that comes again while the follow it ends exists again is now kept once per
follow.

Checked live (scenarios/followsync.sh): Mastodon drops a follow PrivaPub lost, and undoes one it lost itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Federation/Inbox/InboxReceiver.cs

@@ -199,10 +199,27 @@ namespace PrivaPub.Federation.Inbox
 			// true copy carries the same
 			if (!queued && dedupe != default && CarriesOther(await _queue.Payload(dedupe, token), activity))
 				queued = await _queue.Enqueue(JobKind.ProcessInbox, queuedPayload, host, $"{dedupe}|{Digest(activity)}", token);
+			// an Undo of a Follow that comes again while the follow it ends exists again is meant for that follow: Mastodon
+			// gives every Undo it sends after reading a roll-call (FEP-8fcf) one id, `{actor}#follows//undo`. It is kept
+			// once per follow, so a retry still counts as a copy
+			if (!queued && dedupe != default && type == "Undo" && await FollowAgain(activity, actorUri, token) is { } follow)
+				queued = await _queue.Enqueue(JobKind.ProcessInbox, queuedPayload, host, $"{dedupe}|{Digest(activity)}|{follow}", token);
 			_logger.LogInformation("Inbox {Recipient}: {Type} from {Actor} queued", recipient?.Handle ?? "shared", type, actorUri);
 			return new(StatusCodes.Status202Accepted, Reason: !queued ? "duplicate" : forwardedBy != default ? "forwarded" : "queued");
 		}
 
+		// the follow an Undo of a Follow would end, when there is one: its record's id
+		async Task<string> FollowAgain(JsonNode activity, string actorUri, CancellationToken token)
+		{
+			if (activity["object"] is not JsonObject inner || Value(inner, "type") != "Follow" || Id(inner["object"]) is not { } targetUri
+				|| await _localActors.FindByUri(targetUri, token) is not { } target)
+				return default;
+			var follower = await MongoDB.Entities.DB.Default.Find<Follower>()
+				.Match(f => f.ActorURI == actorUri && f.LocalActorId == target.Id && f.LocalActorKind == target.Kind)
+				.ExecuteFirstAsync(token);
+			return follower?.ID;
+		}
+
 		static string HostOf(string uri) => Uri.TryCreate(uri, UriKind.Absolute, out var parsed) ? parsed.Host.ToLowerInvariant() : default;
 
 		static bool CarriesOther(string earlierPayload, JsonNode activity)