Book Cataloging App Help Needed!

Bookish friends I need your help! I need to know how you conveniently keep track of what you have on your physical book shelf. I would like to use a book app on my phone so that I always have my catalog with me, but I need recommendations on which apps are really user friendly and have helpful features. While glorying in my latest grab of book bargains yesterday, I discovered that I really need to have a way of cataloging the books that I own so that I don't double buy. Since I am trying to rebuild my physical library I have accidentally purchased the same book twice several times because while standing with a fifty cents book in my hand I figure it's better to err on the side of caution than not have a book that I've been wanting. I'm not being a good bargain shopper if I'm purchasing more than I need, right? Anyhoo, I found a box of books that I had stuck away, forgotten about, and rediscovered yesterday. And of course, I now have doubles again! (Don't judge!) I really need to keep this from happening again, it's redundant and ridiculous I know! I know that there are apps out there that allow you to scan your books and that's exactly what I would like. I'm going to download a few apps this weekend and give them a try, but I would love to know what you use and what you like and dislike about the app. 

Thanks in advance!
Monica