so first off
make certain the tag is the primary key for the sheep table (so no duplicates can be entered.. but that does mean that you can only enter a sheep once a tag has been defined outside the system and allocated to a sheep
then it comes down to your form design, and that means placing some code behind the form to take various actions
what I suspect you are going to need is to put some code behind the control that contains the sheep tag / Stock No. put it in the controls in update event
that probably needs to be a dlookup
if the row already exists then you need to cancel the current add new record and display the correct row.
im not too sure how you'd cancel the current add new record though
I'd rather be riding on the Tiger 800 or the Norton