Excel’s conditional formatting option has lots of handy uses including highlighting cells that contain duplicate data.
To do this, highlight the range to investigate, for example, a series of entries in column C starting with cell C1. Now choose Format, Conditional Formatting, select Formula Is and enter this in the formula area:
=COUNTIF($C:$C,$C1)>1
Click Format and then select a pattern and color to use and click Ok.
Now duplicate entries in the column will be indicated with the shading you’ve provided. You can copy and paste this formatting down to other cells in column C to include them.