preventing update of customer balance twice

Without seeing your code, it's hard to make specific suggestions.  But one solution is to generate a unique identifier, something like a transaction or order id.  When the user clicks submit, the application should check if payment has already been made for this id.  If so, then it has found a duplicate payment.

how do you make a unique id?  Cannot make a unique id based on time.

You can just use an integer primary key column + auto_increment

This is my code for updating the balance.

<cfquery name="updBal" datasource="#dsn#">
       update cust
       balance = balance - #form.payment#
</cfquery>

<cfquery name="addToTransact datasource="#dsn#">
     insert into transact
     (name, payment, pay_date)
     values
     ('#form.name#', '#form.payment#', #Now()#)
</cfquery>

Transact table and cust table have primary id (transactid and custid)

Sometimes this piece of code is executed twice.  When that happens transactid
is incremented twice for the two records inserted.

How would a primary key come into play?

Thanks.

That might work. Let me try it.
Thanks.