SP Online classic lists: check that value doesn't already exists

Hi,

I have a list with 4 columns and 3 of them take employee names. 

The goal is to avoid having 2 or more rows with duplicated names. Example:

Title Name1 Name2 Name3

In this example "Steve" should not be available to be saved in the list because he is already in the first row. I forced unique values for the name columns but it only check in the column scope so it not works for checking between Name1 and Name2 columns. 

How can I make that kind of check?
Best regards
Americo