diff --git a/app/system/backend/sync_router.py b/app/system/backend/sync_router.py
index 59f13b8..ddccdfc 100644
--- a/app/system/backend/sync_router.py
+++ b/app/system/backend/sync_router.py
@@ -110,6 +110,19 @@ async def sync_from_vtiger() -> Dict[str, Any]:
             if existing:
                 # Link vTiger ID to existing customer
                 current_vtiger_id = existing[0].get('vtiger_id')
+                
+                # Check if this vtiger_id is already assigned to another customer
+                vtiger_owner = execute_query(
+                    "SELECT id, name FROM customers WHERE vtiger_id = %s",
+                    (vtiger_id,)
+                )
+                
+                if vtiger_owner and vtiger_owner[0]['id'] != existing[0]['id']:
+                    # vTiger ID already belongs to another customer - skip
+                    logger.warning(f"⚠️  vTiger #{vtiger_id} allerede tilknyttet '{vtiger_owner[0]['name']}' - springer over '{existing[0]['name']}'")
+                    not_found_count += 1
+                    continue
+                
                 if current_vtiger_id is None:
                     execute_query(
                         "UPDATE customers SET vtiger_id = %s, last_synced_at = NOW() WHERE id = %s",