I received a payment in AR as cash and it should have been a cheque. Can I post it in the GL as a Debit without having a duplicate entry in AR?

In order to assist you with this question please be very specific about the specific data entry steps you are referring to. Thanks

Typically cash and cheques end up debiting the same bank account when they are processed in AR, so you'll have to explain a little more about the problem you are having.