Tax for Requistion

We have been struggling with this issue for a while and want
to find out what is the best practice. I wonder if anyone can help to shed some
light on this.

Our users are entering requisitions inconsistently regarding
"with tax" or "no tax". Some of them enter the total amount (item price + tax) and
apply “no tax”; this is working fine. Some of them enter just the amount
without tax and let the system calculate; this way works fine as well. The
problem is that sometime the user enter the total amount with tax and doesn’t
apply “no tax”, and when the PO was generated, tax will be applied again to cause double taxation.