Two Much by Donald E WestlakeRead about another entry on the list.
Art Dodge invents a twin brother so he can date two different women. It works pretty well until he gets confused, and one of the women thinks her boyfriend's brother has made a pass at her. She insists her boyfriend – Bart – confront his sleazy bro and wants to be there when he does. It's a problem. Of course it's no surprise that Art's losing the plot because he is also trying to stay one step ahead of some mobsters he has mortally offended. Writer Westlake was no stranger to false identities himself having published books under at least 16 names.
--Marshal Zeringue