How to avoid duplicate entries on INSERT in MySQL?

use an autoincrement, you can get the last id issued with the mysql_insert_id property.

If for some reason that's not doable, you can craete another table to hold the last id used, then you increment that in a transaction, and then use it as the key for your insert into the table. Got to be two transctions though, otherwise you'll have the same issue you have now. That can get messy and is an extra level of maintenance though. (reset your next id table to zero when ther are still some in teh related table and things go nipples up quick.

Short of putting an exclusive lock on the table during the insert operation (not even slightly recomended), your current solution just can't work.

Okay expanded answer based on leaving schema as it is.

Option 1 in pseudo code

StartTransaction
try
  NextId = GetNextId(...) 
  AddRecord(NextID...)
  commit transaction
catch Primary Key Violation
  rollback transaction
  Do the entire thing again
end

Obviously you could end up in an infinite loop here, unlikely but possible, probably run out of stack space first.

You could some how queue the requests and then attempt to process them, if successful remove from queue.

BUT make customerid an auto inc the entire problem dispappears. It will still be the primary key, you just don't have to work out what it needs to be any more, in fact you don't supply it in the insert statement, mysql will just take care of it for you.

The only thing you have to remember is if you need the id that has been automatically created is to request it in one transaction.

So your insert query needs to be in the form

Insert SomeTable(SomeColumns) Values(SomeValues)
Select mysql_insert_id

or if multiple statements gets in the way wrap two statements in a start stransaction commit transaction pair.