diff --git a/app/Federation/Handlers/DirectMessageHandler.php b/app/Federation/Handlers/DirectMessageHandler.php
index 2f6a171da..c429e5742 100644
--- a/app/Federation/Handlers/DirectMessageHandler.php
+++ b/app/Federation/Handlers/DirectMessageHandler.php
@@ -74,9 +74,10 @@ class DirectMessageHandler
             $left = DmConversationParticipant::where('conversation_id', $existing->id)
                 ->where('state', DmConversationParticipant::STATE_LEFT)
                 ->pluck('profile_id')
+                ->map(fn ($id) => (int) $id)
                 ->all();
 
-            $readers = $readers->reject(fn (Profile $profile) => in_array($profile->id, $left));
+            $readers = $readers->reject(fn (Profile $profile) => in_array((int) $profile->id, $left, true));
 
             if ($readers->isEmpty() || $this->requestLimitReached($existing, $actor, $readers)) {
                 return null;
diff --git a/app/Models/DmConversation.php b/app/Models/DmConversation.php
index 5cf84a775..1af3015c0 100644
--- a/app/Models/DmConversation.php
+++ b/app/Models/DmConversation.php
@@ -49,7 +49,7 @@ class DmConversation extends Model
     public static function participantsHash(array $profileIds): string
     {
         $ids = array_values(array_unique(array_map('intval', $profileIds)));
-        sort($ids, SORT_NUMERIC);
+        sort($ids);
 
         return hash('sha256', implode(':', $ids));
     }
diff --git a/tests/Feature/DirectMessage/DirectMessageInboundTest.php b/tests/Feature/DirectMessage/DirectMessageInboundTest.php
index e5b000594..10c4b572f 100644
--- a/tests/Feature/DirectMessage/DirectMessageInboundTest.php
+++ b/tests/Feature/DirectMessage/DirectMessageInboundTest.php
@@ -11,6 +11,7 @@ use App\Models\Notification;
 use App\Models\Status;
 use App\Models\UserDomainBlock;
 use App\Models\UserFilter;
+use App\Services\DirectMessageService;
 use Illuminate\Foundation\Testing\LazilyRefreshDatabase;
 use Illuminate\Support\Facades\DB;
 use Illuminate\Support\Facades\Http;
@@ -439,7 +440,7 @@ describe('groups', function () {
     it('drops a message when the only person it could reach here has left the group', function () {
         $bob = dmProfile(dmLocalUser());
         $alice = dmRemoteProfile();
-        $carol = dmRemoteProfile('carol', 'other.example');
+        $carol = dmNeighbour($alice, dmRemoteProfile('carol', 'other.example'));
         dmSeedHosts();
 
         dmDeliver($alice, dmNote($alice, '1', [$bob, $carol]));
@@ -464,6 +465,30 @@ describe('groups', function () {
     });
 });
 
+describe('conversation identity', function () {
+    it('hashes the same people to the same conversation whatever order they come in', function () {
+        $a = 1007571621538590723;
+        $b = $a + 1;
+        $c = $a + 2;
+        expect(DmConversation::dmHash($a, $b))->toBe(DmConversation::dmHash($b, $a))->and(DmConversation::participantsHash([$a, $b, $c]))->toBe(DmConversation::participantsHash([$c, $a, $b]))->and(DmConversation::participantsHash([$a, $b, $c]))->toBe(DmConversation::participantsHash([(string) $b, $c, $a, $a]))->and(DmConversation::dmHash($a, $b))->not->toBe(DmConversation::dmHash($a, $c));
+    });
+
+    it('keeps one conversation when two participants have neighbouring ids', function () {
+        $bob = dmProfile(dmLocalUser());
+        $alice = dmRemoteProfile();
+        $carol = dmNeighbour($alice, dmRemoteProfile('carol', 'other.example'));
+        dmSeedHosts();
+
+        dmDeliver($carol, dmNote($carol, '1', [$alice, $bob]));
+        dmDeliver($alice, dmNote($alice, '2', [$bob, $carol]));
+
+        $mine = app(DirectMessageService::class)->findOrCreateConversation($bob, collect([$alice, $carol]));
+
+        expect(DmConversation::count())->toBe(1)
+            ->and(DmMessage::where('conversation_id', $mine->id)->count())->toBe(2);
+    });
+});
+
 describe('audience', function () {
     it('does not treat public, unlisted or followers-only notes as direct', function () {
         $alice = dmRemoteProfile();
diff --git a/tests/Feature/DirectMessage/helpers.php b/tests/Feature/DirectMessage/helpers.php
index 4c3a33a34..a886c6ad0 100644
--- a/tests/Feature/DirectMessage/helpers.php
+++ b/tests/Feature/DirectMessage/helpers.php
@@ -6,6 +6,7 @@ use App\Models\User;
 use App\Models\UserSetting;
 use App\Util\ActivityPub\Inbox;
 use Illuminate\Support\Facades\Cache;
+use Illuminate\Support\Facades\DB;
 
 if (! function_exists('dmLocalUser')) {
     /**
@@ -42,6 +43,18 @@ if (! function_exists('dmLocalUser')) {
         ]);
     }
 
+    /**
+     * Give $b the id right after $a, the way two profiles created in the same
+     * millisecond end up. Ids that close are equal once compared as floats,
+     * so anything that orders or matches ids has to cope with it.
+     */
+    function dmNeighbour(Profile $a, Profile $b): Profile
+    {
+        DB::table('profiles')->where('id', $b->id)->update(['id' => $a->id + 1]);
+
+        return Profile::findOrFail($a->id + 1);
+    }
+
     /**
      * Seed the DNS and banned-domain caches so URL validation passes without
      * a network lookup. Call after factories, the lazy refresh can flush the