This might work
Sort your data by checknumber and then payment link
Create a formula as "Unique_Amount":
if onfirstrecord then {table.amountfield} else if previous({table.checkNUmberfield})={table.checknumberfield} then 0 else {table.Amountfield}
Place it on your details to validate it is giving the correct data on the correct rows and a 0 on duplicate data.
Use this @Unique_amount formula field to SUM on in your crosstab.
Double check the results.
This assumes you do not have the user changing mid way through any one "set".
Edited by DBlank - 30 Mar 2009 at 10:29am