Okay. You need to add a data connection to the list. Then add a conditional formatting rule on the section - in the first dropdown, select your list data connection and select the field for the username - you'll see with repeating data, the "Select" has a drop down (see screenshot - mine isn't a list data source, I just wanted you to see the drop down because it can be tricky to find) - pick "all occurrences" of your field. Set the condition to be "are not equal to" and set the last drop down to use a formula - use the username function there.
