There is no simple way to fix your problem. There are normally a few reasons for this issue. The customized form is corrupted. The data fields in the check form design are no longer in order. Or the commands on the check form have been deleted. When you open a check form in design the order numbers & command fields are not visible. In order to view these fields at the top of the check form design screen click on Options. Place a check mark next to Commands and click OK. It should show the red "C" command fields. This helps tell the program how to read the lines on check form. If you go back to Options place a check mark next to Order Number and click OK. The order numbers also play a role in how the form is read.

What you may want to do is make 2 or 3 changes at a time and test print. This is to make sure the form is still printing correctly. This will also help to find out what change or changes were made last to cause the check form to print a bunch of copies.

In addition to the earlier post, I believe you may be missing an Object Type "Command" line at the end of your form, with the Object Type description "Read next line item Record".