Visual FoxPro. Creation of a restriction on input

There's a replacement table. It has the id of the old meter and the id of the new meter. It is necessary to prevent the entry as a new id, id counter, which was already indicated in the field id of the old counter. For example, 1 - 2, and then 2 - 1. Or 1 - 2, 2 - 3, 3 - 1.