As i understand, this happens because Account Number is already displayed while the record is not yet saved.. I'm just going to suggest solution to avoid this problem.. :-P
Instead of displaying it immediately, you can put the word "Auto-generated". Then, after saving the record, you can display the generated Account Number..
I disagree with locking method when clicking "New" since only one user at a time would be able to fill up their information.. Furthermore, it would also take time completing the information before they save the record..
I hope this helps.. :-)
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