Songs appearing twice in many mp3 apps, though there is only one file per song

I have reordered my mp3 collection with the excellent easyTag tool. Now on my LG-430 and on my Xperia Z1 compact, in the native music players of the smartphones as well as in the external apps Equalizer+ and Poweramp, in many albums every song appears twice. An example is Mick Jagger's "Goddess In The Doorway". I have checked with a file explorer that there is really only one mp3 file for every song. I have checked with easyTag that all tags are set correctly. Another strange tagging error happens with the Stones' famous "Exile On Main Street" album (OK, I admit, I am a Stones junkie, forgive me). Here the very first song of this early 70ies double LP, "Rocks Off", is put into an album called "Exile On Main St. CD1". All other songs of the double LP are put into an album with exactly the same name. What am I doing wrong or what is wrong with the mp3 collection apps? Greetings from Germany!