How to highlight duplicate values in a list of columns?

I'm looking for a way to highlight duplicate entries in a list of columns.

As an example, A1 to A20 cells - each cell gets values entered manually. If let's say a value entered at A3 is same as a value entered in A17 (as the values are entered over time, not all at one time) I'd like to highlight the cell in red. I've tried with conditional formatting but haven't found a workaround for this problem.

Thanks in advance for all help in this issue.