In Excel: How can I check to see if a cell matches another cell in two different columns, and if it is true, display the value of another

In other words, say I have a column with 100 values. To the right, I have another column with only 20/100 values. How can I check to see if the cells have duplicates, and if so, return a different value that is in the same row of the first column.

For example:

  ____A______B_______C_______D____
1 |Larry | 11111 | 22222 | Bob   |
2 |Bob   | 22222 | 44444 | Steve |
3 |Mike  | 33333 | 55555 | Jim   |
4 |Steve | 44444 |       |       |
5 |Jim   | 55555 |       |       |
  ________________________________

In the above diagram, Column C checks to see if there is a duplicate value in Column B, and if there is, it returns the corresponding name in Column A to Column D.