Being duplicitous is a mixed bag. The most obvious effect is that I created a pattern of loosening up my values when nobody was watching. They were still my values, and I still believed in them... but repeatedly edging across the border of behavior's acceptable limit (and never getting caught) allowed me to feel increasingly comfortable with things I should have been ashamed of. Basically, I numbed up my conscience because I was getting all the rewards and no notable negative consequences.
When no one was watching I looked at porn. When no one was watching I considered the possibility of meeting someone for anonymous sex. I started looking at online personals. Somehow, despite that I'm a principled person, I got myself to throw everything I believed temporarily away while I flirted with arousing alternative options. I lost track of what was really off limits by repeatedly violating my own standards. Would I then stop with looking? Would I stop with kissing? Would I stop anywhere? Yes, probably, but nothing was certain when I was (am?) my other self.
There have been some good things from duplicity. I didn't throw away my faith in the name of being "authentic". Sure, avoiding duplicity is a good thing, but not necessarily if the approach is to kill the virtuous part of myself and declare the licentious part the "real" me. People still treat me as if I am a principled person, so I believe them. And, yes, my mistakes represent the "real" me in a sense, but not the me I want to become or the me I believe that I am most fundamentally. Perpetually reinventing myself as a better person rather than accepting whatever "realities" may be is the province of repentance and a principle that I try to let guide my life.
For better or for worse, I don't want other people to define me. I want to define myself. And because I know the self I want to be, it would be nice if I had the personal integrity and strength to always behave the same way when I'm alone that I would if my parents were standing next to me.