Duplicate check over several Workbook

I am checking duplicate entries on a speadsheet using the array:
if(Countif($A$1:A2,A2)>1,"Duplicate","")

Where data for checking is in Col A starting at A2

This works very well, but I now need to check against another worksheet as well, with data in same Col

any ideas would be appreciated

great forum

One thing I did not say was I am on a Mac and I am not sure if an add-in will work.

Thanks for the quick response.