Buying an album from Google Play of which I already have some songs I want to buy an album, but I already have some songs from it which I bought from the Play Store. Will there be duplicates of the ones I already have? Also, will some money be deducted from the cost of the remaining album like on iTunes, or will I have to pay the cost of the whole album? Thank you!
In my experience... Yes, you will be charged full price for the album even if you already have some songs on that album.  You will not have duplicate songs in your library if the copy of the songs you previously had were from that exact same album and also purchased from Google Play.

However, if the songs you already had were from a different album than the one you are purchasing (eg. a regular album vs deluxe edition album vs a single or an artist album vs a soundtrack with the same song on it), then you will end up with a duplicate copy of that song in your library even if both were purchased from Google.

You'll also get duplicates in your library if the songs are from the same album but from a different source like iTunes, Amazon or ripped from a personal CD and uploaded to Google Music, but in your case you already said they were purchased from Google Play so this added comment is for others that might have this situation.