Is it my imagination, or are 2 or 3 different actresses playing Aly ?

It's not your imagination. Allison has been played by three different actresses. Krysten Ritter, Lindsey Broad, and Laura Clery.

I think they switched the actress again to screw with doug's thinking of that he's living in a sitcom!!!

My favorite is the one from the army show that got cancelled a few years ago.