Excel compare two columns and highlight duplicates

A simple formula to use is

=COUNTIF($B:$B,A1)

Formula specified is for cell A1. Simply copy and paste special - format to the whole of column A

There may be a simpler option, but you can use VLOOKUP to check if a value appears in a list (and VLOOKUP is a powerful formula to get to grips with anyway).

So for A1, you can set a conditional format using the following formula:

=NOT(ISNA(VLOOKUP(A1,$B:$B,1,FALSE)))

Copy and Paste Special > Formats to copy that conditional format to the other cells in column A.

What the above formula is doing:

• VLOOKUP is looking up the value of Cell A1 (first parameter) against the whole of column B ($B:$B), in the first column (that's the 3rd parameter, redundant here, but typically VLOOKUP looks up a table rather than a column). The last parameter, FALSE, specifies that the match must be exact rather than just the closest match.
• VLOOKUP will return #ISNA if no match is found, so the NOT(ISNA(...)) returns true for all cells which have a match in column B.