an IF formula, probably

I'm wondering how I can ensure that the smae number in a log is never used twice by accident.

For example, I have created quite an info intensive log file with each entry having a unique reference number (ideally in numbering order from 101, 102, 103 ... on up). I just came across two entries in which the same reference number was used accidentally.

Now if there is a conditional IF formula I can use that would be the easiest I think. But as the log entries grow I need the IF formula to be able to look @ cells A'n' (last refernece number used) thru to cell A1 (very first reference number used, 101) & ensure that the new reference number being used hasn't already been previously used.

I know the answer is probably going to be very obvious when I see it, so thanx in advance.

cheers