There are three main metadata fields to be aware of when organizing your Google Play music library:'ARTIST', 'ALBUM ARTIST' and 'ALBUM'.
Google Play interprets different text in the 'ALBUM ARTIST' field as separate albums. If the same album is appearing more than once, you can usually resolve the issue by editing the 'ALBUM ARTIST' field so that the artist is listed consistently across the songs you'd like to add to a particular album (see Editing metadata and album art).
If the 'ARTIST' and 'ALBUM' fields are set but 'ALBUM ARTIST' is blank, Google Play will use the 'ARTIST'and 'ALBUM' fields to determine the album that the song should be added to. If only the 'ALBUM' field has been set, multiple albums that use the same title may be combined ('Greatest Hits' for example).