fix duplication of same objects in db with when saving a child scenario

src/Model/HydraulicStructures/HydraulicStructures.py

@@ -274,6 +274,9 @@ class HydraulicStructure(SQLSubModel):
         return hs
 
     def _db_save(self, execute, data=None):
+        if not self.must_be_saved():
+            return True
+        
         execute(
             "DELETE FROM hydraulic_structures " +
             f"WHERE pamhyr_id = {self.pamhyr_id} " +