You can limit the entries using Data Validation. Say your range of
invoice #'s are in A1:A2000 (this includes an allotment of empty cells
for future input of invoice numbers). In cell A1 click on, Data and
select Validation. On the settings tab, change the Allow option to
Custom. In the formula bar type the following formula:

=COUNTIF($A$1:$A$2000,A1)1=FALSE

Click OK.

Copy Cell A1. Highlight the rest of your range A2:A2000. Edit, Paste
Special, Validation. OK.

This will only allow unique entries to be entered in that range.

Does that help?

Steve


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