Trigger event for personalization at Saving PO receipt

Hi,

I am facing issue, where if I user trigger event as 'WHEN-VALIDATE-RECORD' and Trigger Object 'RCV_TRANSACTION' system will show desired error at each PO shipments which appear while creating receipt. So if there are large number of lines, system will show error number of times equal to number of shipments. So if I put Trigger Object as 'RCV_CONTROL' system shows correct message when user tries to save the receipt. But in this case, if user clicks 'Header' button, the personalization message goes in loop. Hence, user needs to close the form and start over.

Which is the appropriate event for such requirement of personalization?