I'm having the same issue. "I used substrings" was a little vague but enough to go on and do a little research.
What I'm doing is setting a People picker with a rule based on the selection of another piece of data. I get the double commas.
In the rule I set the Field which is a lookup to a secondary data source with some filtering then the text looking for in this case ",," and what I wanted to translate it to "," using the Substring Translate function.
So, My Rule Value looked like this translate(DisplayName[Project_Number = Project Number], ",," , ",") This did not accomplish what I thought it would, still returned the double comma.
Could you elaborate on what you did a little bit more? Thanks.