two transactions enetered in a table have a difference of f

Two transactions entered in a table have difference of few milliseconds .I would like to restrict the entry of such a transaction.This occurs when two threads are running together.In other words I want keep a check on the uncommitted data running parallely.
I have tried with UR,CS,RS.Nothing seems to work.

it would be great if anyone could help me out on this.

Thanks in advance,