Data subsets and duplicates

Hello,

I have been trying to find the answer in a few forums but haven't found quite what I'm looking for.

I have a column (A) of around 2000 numbers, each of which is linked to another value (ie: A1 = 123456 which is linked to B1 = 3). All of the values in column A are unique, as are those found in a third column (C), but all 500 of the values found in column C are found in column A. What I'd like to do is find where the value in column C is found in column A (ie: a duplicate) and thus the value in column B so I can fill out a fourth column (D) with the value found in B.

I really hope that explains things - perhaps due to the specific nature of the query I've been unable to find an answer elsewhere so apologies if this has been covered before.

Many thanks.