Disable Saving A Transaction When Other User's Saving A Transaction

Hallo.I have a personalization in receipt number (aging receivable) so that the receipt number increments automatically.
I get the last receipt number, then increments by 1.
I put the personalization in 'WHEN-VALIDATE-RECORD' trigger.
But, there is a possibility that the receipt number is the same if there are 2 users saving the transaction at the same time.

Could anybody know how to disable saving a transaction when other user is also saving a transaction?

Thanks