Will two albums duplicate pictures in Photos.app? I am trying to put together several albums and some of them have a picture from another album. For example a parties album and a birthday 2015 album or a granddaughters 2015 album.  So how do I put the same photos in these 3 albums, and are they duplicated or just cross referenced?
Just drag the photo from one album to another. The photo will be added to the second album and not removed from the first. Only one master copy of the photo is kept and is cross-referenced throughout the library regardless of where it is used.