Comparing two columns for duplicates

I'm trying to get a comparison of two different columns to find duplicate values that are only duplicated in one of the columns. I have a dataset that has lots of duplicate entries in the Database field as we have multiple sites on each database. I'm trying to identify and filter out any sites that have already migrated onto the newer version based on this data. I have a list of the database names that are on the new site but I need to know how I can highlight the databases in column A that are showing up in column C. I've tried various attempts at conditional formatting for duplicates, this highlights any duplicates in the sheet which is not helpful, and formulas to look in column A and if the values in A are in column C show true. I can get it to work on a single cell but as soon as I copy it down it doesn't work. The formula I was attempting to use is =IF(A2=C:C,TRUE) and then drag that down. It shows true for A2 which does have a duplicate but it doesn't show any more True values for any other matches.

I've attached a sample workbook which is representative of what I'm trying to do. I've recreated my attempt at an if statement and you can see how when it was copied down it did not work.

Any help with this would be hugely appreciated

Try this:

=IF(ISNA(MATCH(A2,$C$2:$C$13,0)),FALSE,TRUE)

You're welcome.