User Exit in Sales Order for line item change

Please help me with this requirement. Need to give a pop message when there is a change in quantity of a line item in sales order if satisfies some conditions on press of enter. I did the changes in user exit check_vbap.

But this user exit is getting executed twice for those line items whose quantity has been changed.

It is executing in this manner. First it will execute for changed line items then gets execute for all the line items present in SO.

Please suggest how to give a pop message only once for changed line items on press of enter.