preventing update of customer balance twice

I have a web application using coldfusion and mysql. There is a form where
customer's payment is entered and submitted. Upon submission customer's
balance is updated. Sometimes customer's balance is updated twice.
How do I prevent this from happening?  

Example:
Balance:  1000
payment: 100
After update: balance = 800
 
Thanks.
SonnyReyAsked:
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johanntagleCommented:
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.
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SonnyReyAuthor Commented:
how do you make a unique id?  Cannot make a unique id based on time.
0
johanntagleCommented:
You can just use an integer primary key column + auto_increment
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SonnyReyAuthor Commented:
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.
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johanntagleCommented:
transactid is the primary key for transact, and yet the insert statement above does not include that column, so I assume that has integer auto_increment in your definition.  I think what you need to do is to already insert a row to the transact table before processing payment (so payment column is currently null), note the generated transactid, then make it part of the form, maybe as a hidden field.

Then when the form is submitted, a check is made if the row with that transactid already has a value for payment.  If so, then it is considered a duplicate and nothing else happens.  If not, then the usual balance update should proceed.
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SonnyReyAuthor Commented:
That might work. Let me try it.
Thanks.
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