Probably does not impact the price much, if at all. I would not pay a penny more for a jersey that's signed twice. But then again, I am not a huge fan of signed game-used equipment in general. I just don't love it when the ink writing gets too "busy" as it tends to detract from the game-used article itself.
I can accept it more with Bats or Balls, but autographs on fabric jerseys and caps do not do a thing for me. I know I'm wandering from your specific question, and more into personal preferences. But I've seen many jerseys signed front and back at auction, and have never seen those with a duplicate signature sell for any more than normal.
Perhaps others here might feel differently, but I kind of doubt it.
