A follow request sent again carries a new id

Lemmy keeps the ids of the activities it received and never answers one again, so resending the same Follow could not
heal a follow whose Accept it lost: the village's community follow stayed pending through two resends. Each resend is
now the same follow under its own id (<follow id>-again-<n>), and an Accept naming any of them answers the follow;
the Undo still embeds the follow, so servers match it by its actor and object. Sent this way, the stuck follow was
accepted at once.

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

FEDERATION.md

@@ -230,9 +230,10 @@ Posts with a location (shown to nearby users of this server) never leave the ser
   is read again from its author's server, a deletion of a public or unlisted post is applied once that server answers
   404 or 410, and anything else is dropped. LD signatures are not verified. A reply forwarded this way is kept when
   someone here follows the author of the post it answers, as Mastodon does.
-- **Follow requests** still unanswered are sent again after 15 minutes, an hour, 6 hours, a day, two and four days, the
-  same activity each time: a server can take a Follow and lose its answer (Lemmy 1.0 sends nothing it queued for a
-  server before it started sending there), and one that holds the follow already answers the copy.
+- **Follow requests** still unanswered are sent again after 15 minutes, an hour, 6 hours, a day, two and four days: a
+  server can take a Follow and lose its answer (Lemmy 1.0 sends nothing it queued for a server before it started
+  sending there), and one that holds the follow answers it again. Each is the same follow under a new id
+  (`…/follow-<id>-again-<n>`), since Lemmy ignores an id it has seen; an answer naming any of them counts.
 - **Fetching.** All fetches are signed by the instance actor. They go only to public addresses, follow at most three
   redirects and read at most 1 MB.
 - **Threads.** A reply's missing parents are fetched, up to 10 levels. When someone here opens a public remote thread,

PrivaPub.Tests/Federation/InboxGapTests.cs

@@ -68,12 +68,18 @@ namespace PrivaPub.Tests.Federation
 			Assert.Equal(0, await After(TimeSpan.FromMinutes(30)));
 			Assert.Equal(1, await After(TimeSpan.FromMinutes(77)));
 			Assert.Equal(3, await Follows());
-			Assert.All(await _harness.Outgoing(quiet.Id + "/inbox"), a => Assert.Equal(following.FollowActivityURI, a["id"]!.GetValue<string>()));
+			// each one again under its own id (Lemmy never answers an id it has seen), and an answer naming it counts
+			var sentIds = (await _harness.Outgoing(quiet.Id + "/inbox")).Select(a => a["id"]!.GetValue<string>()).ToList();
+			Assert.Equal(new[] { following.FollowActivityURI, following.FollowActivityURI + "-again-1", following.FollowActivityURI + "-again-2" }, sentIds);
 
 			// a request older than the count is sent again too; an answered one never
 			await DB.Default.Update<Following>().MatchID(following.ID).Modify(b => b.Unset(f => f.Resent)).Modify(b => b.Unset(f => f.ResentAt)).ExecuteAsync(token);
 			Assert.Equal(1, await After(TimeSpan.FromDays(1)));
-			await DB.Default.Update<Following>().MatchID(following.ID).Modify(f => f.State, FollowState.Accepted).ExecuteAsync(token);
+			await _harness.Deliver(quiet, "/human-centipede", new JsonObject
+			{
+				["id"] = NewId(quiet, "accepts"), ["type"] = "Accept", ["actor"] = quiet.Id, ["object"] = following.FollowActivityURI + "-again-2"
+			});
+			Assert.Equal(FollowState.Accepted, (await DB.Default.Find<Following>().OneAsync(following.ID, token)).State);
 			Assert.Equal(0, await After(TimeSpan.FromDays(30)));
 		}
 

PrivaPub/Domain/Social/FollowService.cs

@@ -298,7 +298,10 @@ namespace PrivaPub.Domain.Social
 				var inbox = following.TargetInboxURL ?? (await _remoteActors.GetActor(following.TargetActorURI, refresh: false, token))?.InboxURL;
 				if (follower != default && !string.IsNullOrEmpty(inbox))
 				{
-					await _delivery.Enqueue(follower, new[] { inbox }, FollowActivity(follower, following), token,
+					// under a new id each time: Lemmy keeps the ids it received and never answers one again
+					var again = FollowActivity(follower, following);
+					again["id"] = AgainId(following.FollowActivityURI, following.Resent + 1);
+					await _delivery.Enqueue(follower, new[] { inbox }, again, token,
 						again: "resend-" + now.ToString("yyyyMMddHHmm", System.Globalization.CultureInfo.InvariantCulture));
 					sent++;
 				}
@@ -308,6 +311,11 @@ namespace PrivaPub.Domain.Social
 			return sent;
 		}
 
+		// a follow sent again is the same follow under a new id, which an answer may name (AcceptHandler.FindFollowing)
+		public const string Again = "-again-";
+
+		public static string AgainId(string followActivityUri, int resend) => $"{followActivityUri}{Again}{resend}";
+
 		static JsonObject FollowActivity(LocalActor follower, Following following) => new()
 		{
 			["@context"] = ActivityPubRenderer.ActivityStreams,

PrivaPub/Federation/Inbox/Handlers/AcceptHandler.cs

@@ -53,7 +53,10 @@ namespace PrivaPub.Federation.Inbox.Handlers
 		{
 			if (follow is JsonObject && Value(follow, "type") is { } type && type != "Follow")
 				return default;
-			var followId = Id(follow);
+			// (a follow sent again names the first one under a new id: FollowService.AgainId)
+			var followId = Id(follow) is { } named && named.IndexOf(Domain.Social.FollowService.Again, StringComparison.Ordinal) is var at and > 0
+				? named[..at]
+				: Id(follow);
 			var byId = followId == default
 				? default
 				: await dbEntities.Followings.Match(f => f.FollowActivityURI == followId && f.TargetActorURI == target.ActorURI).ExecuteFirstAsync(token);

docs/INTEROP.md

@@ -533,6 +533,9 @@ What it showed:
 - **Every bare `Announce{object}` is answered 400** (`Failed to parse object`: Lemmy dereferences it expecting an
   activity), as Lemmy answers the compatibility `Announce(Page)` it sends itself. Both 400s show up as dead deliveries
   in the statistics.
+- **It keeps the ids of the activities it received and never answers one again.** A follow it took and whose Accept it
+  lost (sent before its worker for our server started) stayed pending through two resends of the same Follow; sent
+  under a new id (2026-10-05), it was accepted at once (the village's community follow, pending since 05:06).
 - **Votes travel only to the community**, which relays them as `Announce{Like}` and `Announce{Dislike}`: counted since
   ed08f80, and a vote turned the other way replaces the first since bbeeda7. A moderator's removal, lock and ban come
   the same way and are applied since 2026-10-05 (the removal was applied all along; the scenario gave up before Lemmy's