repeat output

When I want to make a small change to an existing PO that has already been sent to the vendor/myself, the Messages tab has automatically generated repeated outputs. Is there a way to turn this off so that I have to manually enter that I want the PO to be resent to the vendor. It confuses the vendors sometimes when I sent them a corrected PO and they think it's a new order. I would like to have to manually select "repeat output." Thank you.