Guard directional ticket link writes

tests/Feature/Projects/ProjectTicketLinkControllerTest.php

@@ -146,6 +146,66 @@ class ProjectTicketLinkControllerTest extends TestCase
             ->assertSessionHasErrors('linked_ticket_id');
     }
 
+    /**
+     * Ensure relinking an existing reverse-direction pair updates the existing row instead of creating a duplicate.
+     */
+    public function test_linking_an_existing_reverse_direction_pair_reuses_the_same_row(): void
+    {
+        $user = User::factory()->create();
+        $project = Project::factory()->create([
+            'key' => 'CORE',
+        ]);
+
+        app(InitializeProjectTicketMetadata::class)->handle($project);
+
+        $priority = $project->ticketPriorities()->firstOrFail();
+        $taskType = $project->ticketTypes()
+            ->where('is_parent', false)
+            ->orderBy('position')
+            ->firstOrFail();
+        $ticketLinkType = TicketLinkType::factory()->for($project)->create([
+            'label' => 'Blocks',
+            'counterpart_label' => 'Blocked by',
+            'position' => 1,
+        ]);
+        $ticket = Ticket::factory()->create([
+            'project_id' => $project->id,
+            'sequence_number' => 1,
+            'ticket_key' => 'CORE-1',
+            'ticket_priority_id' => $priority->id,
+            'ticket_type_id' => $taskType->id,
+        ]);
+        $linkedTicket = Ticket::factory()->create([
+            'project_id' => $project->id,
+            'sequence_number' => 2,
+            'ticket_key' => 'CORE-2',
+            'ticket_priority_id' => $priority->id,
+            'ticket_type_id' => $taskType->id,
+        ]);
+        $existingLink = TicketLink::factory()->create([
+            'source_ticket_id' => $linkedTicket->id,
+            'target_ticket_id' => $ticket->id,
+            'ticket_link_type_id' => $ticketLinkType->id,
+        ]);
+
+        $this->syncProjectPermissions($project, $user, ['edit', 'view']);
+
+        $this->actingAs($user)
+            ->post(route('projects.tickets.links.store', [$project, $ticket]), [
+                'linked_ticket_id' => $linkedTicket->id,
+                'ticket_link_type_id' => $ticketLinkType->id,
+            ])
+            ->assertRedirect(route('projects.tickets.show', [$project, $ticket]))
+            ->assertSessionHas('status', 'Blocks link was saved.');
+
+        $this->assertSame(1, TicketLink::query()->count());
+
+        $existingLink->refresh();
+
+        $this->assertSame($ticket->id, $existingLink->source_ticket_id);
+        $this->assertSame($linkedTicket->id, $existingLink->target_ticket_id);
+    }
+
     /**
      * Ensure users with edit permission can remove ticket links.
      */