How can I filter a column to have only one instance of repeating values?

Thanks to any who can help...

I have a set of weekly reports that I concatenate into one monthly report. Because individuals may appear in one weekly report or in all of the weekly reports, there may be from one to five rows with their data.

I need to know how many individuals are listed in the weekly reports at the end of the month but I need to weed out the duplicate info from the concatenated report.

Each individual has a unique id number.

How can I filter the id column and cause it to keep only the first instance of each unique id?

This only works with numbers (I think) Assuming your ID numbers are in column A

=COUNT(IF(FREQUENCY(A:A,A:A),1))


Hope this Helps,
Michael