This happened to me too. As you suggested, I'm assuming this is from all the "January Photo Stream", etc., albums. Unfortunately, at this point, there's no good way to remove the duplicates, other than manually.
Also, none of my videos play in Photos for OS X - they all have no thumbnail (blank white) and when I open one it's a black screen, and if I click play the timer/download icon at the bottom right goes round maybe quarter of the way, then stops, then an exclamation mark goes in it.
Not an error - you can continue to work with your old iPhoto library but none of the changes or additions you make to that library will be reflected in Photos, and vice versa.