Is it possible you are adding the event handler twice?  That the code to
add to the Click event has been added twice?

   If you place a breakpoint in the AddCustomer_Click method, does it get
called twice?

You didn't post anything that could be used to identify the problem.  You  
say you checked things, but if you'd checked everything you needed to and  
not missed anything, you'd have found the problem and wouldn't be posting  
here.  So either there's something you didn't check, or there's something  
you missed in something you did check.  Either way, without posting the  
things that need checking, there's no way for anyone else to tell you  
what's wrong.

That said: if the records is getting saved twice, obviously some code that  
saves the record is getting executed twice.  The first thing you need to  
do is figure out what's getting executed twice.  Set a breakpoint at the  
part of your code closest to the database and see how many times you hit  
it for a given button click.

If it gets hit just once, then the problem is somewhere in your database  
side.  If it gets hit twice, then there's something wrong with the code  
side.  Even better, you can look at the call stack for each time it gets  
hit and get an idea for why it's getting called each time.  One time will  
look like what you expect.  The other won't.  The one that isn't what you  
expect is where you have a problem.

Your post implies that you're not writing the code to add the Click event  
handler yourself.  If it weren't for that, the thing I'd be most likely to  
suspect is that you're adding the event handler twice.  If I've  
misinterpreted the implication, then IMHO that's the first thing you  
should look for: how many times are you subscribing the event handler?  It  
should only be once, but if it's twice you'll find it getting executed  
twice for each Click event.

Pete