diff --git a/PrivaPub/Infrastructure/Statistics/InteractionLedger.cs b/PrivaPub/Infrastructure/Statistics/InteractionLedger.cs
index 1ce8843..4b405e8 100644
--- a/PrivaPub/Infrastructure/Statistics/InteractionLedger.cs
+++ b/PrivaPub/Infrastructure/Statistics/InteractionLedger.cs
@@ -203,13 +203,13 @@ namespace PrivaPub.Infrastructure.Statistics
 				if (host == Interactions.Unknown || _touchedAt.TryGetValue(host, out var at) && now - at < TouchInterval)
 					continue;
 				_touchedAt[host] = now;
-				await DB.Default.Update<RemoteInstance>()
+				await Upsert(() => DB.Default.Update<RemoteInstance>()
 					.Match(i => i.Host == host)
 					.Modify(i => i.Seen, "touched")
 					.Modify(i => i.LastSeenAt, now)
 					.Modify(b => b.SetOnInsert(i => i.FirstSeenAt, now))
 					.Option(o => o.IsUpsert = true)
-					.ExecuteAsync(token);
+					.ExecuteAsync(token));
 				_knownHosts.Add(host);
 				if (_queue != default && _blocks?.IsSuspended(host) != true)
 					await _queue.Enqueue(JobKind.DescribeInstance, host, host, Federation.Objects.InstanceDescriber.DedupeKey(host, now), token);
@@ -270,17 +270,31 @@ namespace PrivaPub.Infrastructure.Statistics
 			return _knownHosts;
 		}
 
+		// two flushes upserting a new day's document at once: one inserts it and the other's insert fails on the unique
+		// key; the second try finds the document and adds to it (MongoDB retries this only for single-document updates)
+		static async Task Upsert(Func<Task> write)
+		{
+			try
+			{
+				await write();
+			}
+			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
+			{
+				await write();
+			}
+		}
+
 		async Task FlushCounters(CancellationToken token)
 		{
 			foreach (var key in _reads.Keys.ToList())
 			{
 				if (!_reads.TryRemove(key, out var count) || count == 0)
 					continue;
-				await DB.Default.Update<InstanceDay>()
+				await Upsert(() => DB.Default.Update<InstanceDay>()
 					.Match(d => d.Day == key.Day && d.Host == key.Host)
 					.Modify(b => b.Inc($"{nameof(InstanceDay.Reads)}.{key.Key}", count))
 					.Option(o => o.IsUpsert = true)
-					.ExecuteAsync(token);
+					.ExecuteAsync(token));
 			}
 
 			var day = DateTime.UtcNow.Date;
@@ -291,22 +305,22 @@ namespace PrivaPub.Infrastructure.Statistics
 			var server = _server.Keys.ToList();
 			if (server.Count == 0 && written == 0 && failed == 0 && newlyDropped == 0)
 				return;
-			var update = DB.Default.Update<ServerDay>()
+			await Upsert(() => DB.Default.Update<ServerDay>()
 				.Match(d => d.Day == day)
 				.Modify(b => b.Inc(d => d.LedgerWritten, written))
 				.Modify(b => b.Inc(d => d.LedgerFailed, failed))
 				.Modify(b => b.Inc(d => d.LedgerDropped, newlyDropped))
-				.Option(o => o.IsUpsert = true);
-			await update.ExecuteAsync(token);
+				.Option(o => o.IsUpsert = true)
+				.ExecuteAsync(token));
 			foreach (var key in server)
 			{
 				if (!_server.TryRemove(key, out var count) || count == 0)
 					continue;
-				await DB.Default.Update<ServerDay>()
+				await Upsert(() => DB.Default.Update<ServerDay>()
 					.Match(d => d.Day == key.Day)
 					.Modify(b => b.Inc($"{key.Field}.{key.Key}", count))
 					.Option(o => o.IsUpsert = true)
-					.ExecuteAsync(token);
+					.ExecuteAsync(token));
 			}
 		}