Is this when printing from the spec designer, or a real check? If this is just from the designer, the situation may just be that your check spec has 2 detail sections, one for use when printing payments and the other when printing cash invoices. Should this be your situation, then you can safely ignore this, as only one section prints for real checks.

If this is for a real payment or cash invoice, then the solution is as Doug suggests, to edit the check spec and ensure that the section height of the detail section is only one row, or about 0.1667 inches (6 lines per inch).