[FIX] stock_manual_transfer: Prevent creation of duplicate transfers

To prevent any potential duplication, this fix introduces a
return-early pattern. Now, transfers are not created if the manual
transfer record has already been published.

Issue reported on helpdesk ticket #25879

stock_manual_transfer/models/stock_manual_transfer.py

@@ -66,6 +66,8 @@ class StockManualTransfer(models.Model):
     )
 
     def action_validate(self):
+        if self.state != "draft":
+            return
         route = self.route_id
         if self.warehouse_id not in route.rule_ids.warehouse_id:
             raise ValidationError(_("The selected route doesn't have configured rules on the selected warehouse."))