Lol, I also recall buying multiple copies of a book because I've forgotten I already had it (ran out of shelf space a long, long time ago).
I started reading digital on Stanza for iOS and when that was discontinued (thank you Amazon for acquiring and then killing off competition), I'm glad to have found Marvin which I've been using ever since. I used Stanza for, hmm, 4-5 years? I believe I've been using Marvin for nearly as long.
I've never liked Kindle or iBooks apps for reading. Too restrictive and limited particularly for sideloaded content. At the very least, I need the option to customize tap areas (tap on either side for next page) and disable page turn animations.
Apps and devices are a means to an end. Sure, I'd check new ones occasionally but I'm not gonna dedicate my time obsessing trying to find the perfect reading device/app. Good enough works for me.