diff --git a/src/GmRelay.Web/Services/SessionService.cs b/src/GmRelay.Web/Services/SessionService.cs
index 158fc59..815512f 100644
--- a/src/GmRelay.Web/Services/SessionService.cs
+++ b/src/GmRelay.Web/Services/SessionService.cs
@@ -541,12 +541,18 @@ public sealed class SessionService(
 
         var session = await conn.QuerySingleOrDefaultAsync<dynamic>(
             """
-            SELECT id, max_players AS MaxPlayers, allow_direct_registration AS AllowDirectRegistration
-            FROM sessions
-            WHERE id = @SessionId
-            FOR UPDATE
+            SELECT s.id, s.max_players AS MaxPlayers, s.allow_direct_registration AS AllowDirectRegistration
+            FROM sessions s
+            JOIN game_groups g ON g.id = s.group_id
+            WHERE s.id = @SessionId
+              AND s.is_public = true
+              AND g.public_schedule_enabled = true
+              AND g.public_slug IS NOT NULL
+              AND s.scheduled_at > now() - interval '4 hours'
+              AND s.status <> @Cancelled
+            FOR UPDATE OF s
             """,
-            new { SessionId = sessionId },
+            new { SessionId = sessionId, Cancelled = SessionStatus.Cancelled },
             transaction);
 
         if (session is null || !(bool)session.allowdirectregistration)
@@ -557,39 +563,21 @@ public sealed class SessionService(
 
         var playerId = await _UpsertPlayerAndGetIdAsync(conn, platform, externalUserId, displayName, null, transaction);
 
-        var existing = await conn.ExecuteScalarAsync<bool>(
-            """
-            SELECT EXISTS (
-                SELECT 1 FROM session_participants
-                WHERE session_id = @SessionId AND player_id = @PlayerId
-            )
-            """,
-            new { SessionId = sessionId, PlayerId = playerId },
-            transaction);
+        var registrationStatus = SessionCapacityRules.DecideJoinStatus(
+            (int?)session.maxplayers,
+            await conn.ExecuteScalarAsync<int>(
+                """
+                SELECT COUNT(*) FROM session_participants
+                WHERE session_id = @SessionId AND is_gm = false AND registration_status = @Active
+                """,
+                new { SessionId = sessionId, Active = ParticipantRegistrationStatus.Active },
+                transaction));
 
-        if (existing)
-        {
-            await transaction.RollbackAsync();
-            return false;
-        }
-
-        var activeCount = await conn.ExecuteScalarAsync<int>(
-            """
-            SELECT COUNT(*)
-            FROM session_participants
-            WHERE session_id = @SessionId
-              AND is_gm = false
-              AND registration_status = @Active
-            """,
-            new { SessionId = sessionId, Active = ParticipantRegistrationStatus.Active },
-            transaction);
-
-        var registrationStatus = SessionCapacityRules.DecideJoinStatus((int?)session.maxplayers, activeCount);
-
-        await conn.ExecuteAsync(
+        var inserted = await conn.ExecuteAsync(
             """
             INSERT INTO session_participants (session_id, player_id, is_gm, rsvp_status, registration_status)
             VALUES (@SessionId, @PlayerId, false, @Pending, @RegistrationStatus)
+            ON CONFLICT (session_id, player_id) DO NOTHING
             """,
             new
             {
@@ -600,6 +588,12 @@ public sealed class SessionService(
             },
             transaction);
 
+        if (inserted == 0)
+        {
+            await transaction.RollbackAsync();
+            return false;
+        }
+
         await transaction.CommitAsync();
         return true;
     }