Thanks to all for your replies.
If locking and unlocking is the way to fix this then i will follow the same. I do not maintain any number range,just increment the request number by one from the database.
@ Andrei - "Before a user saves the data in your Z table, call FM NUMBER_GET_NEXT. This should not fail in case two users hit SAVE button at the same time."
Do you mean to say then by using this FM i do not need to lock and unlock the table ?
Thanks.