Autonumbering Problem

I have two forms that populate one table. This table’s primary key generates an autonumber. Everything works fine (ie: a new number is generated for each new entry) except when I go to close the form after the entry. Doing so generates a prohibiting error message:

“The changes you requested to the table were not successful because they would create duplicate values in the index, primary key, or relationship. Change the data in the field or fields that contain duplicate data, remove the index, or redefine the index to permit duplicate entries and try again.”

I set this up this way because I am filling sender and receipient addresses references that draw from the same table. What am I doing wrong? Is there a better way to do this?