Malcolm,
You could try using Data Validation.
Assuming the data to be entered will be in Column B you should click in the
first row of that data and open the Data Validation Function from the
Menubar and select Custom and then in the Formula field enter:
=1=COUNTIF($B$1:$B$2000,B1)
This assumes the data will go down as far as row 2000 and starts at row 1.
(Change as required.)
Then in the Error Alert tab of the Data Validation Dialog you should enter
something like:
This Invoice Number has already been entered!
Make sure that the Style in this tab is selected to be Stop.

When that part is completed, copy that cell and select all the cells below
it and choose PasteSpecial from the menu Bar and click Validation. This
will apply the Data Validation to all the cells you have selected in column
B.