Tax Payer ID

Hi,

1. We have Customer (ABC) with type Organization created in AR. We process Transactions/Receipts on this customer every month

2. Over a period of time, ABC also becomes our supplier.

3. AP dept creates a supplier with name ABC (matches with what is created in AR). When they try to enter the Tax Payer ID at header level they get the error saying that this Tax Payer ID already exists.  However, if we enter the same at Site level we dont get any error ?

How can we over come this issue ? What are the best solutions for this ?

Can we have the Tax ID's created at AP Site level ? What kind of implications will it have ?