Duplicate cards: Any duplicate cards you receive will help progress your coin collection. Depending on the rarity of the duplicate, what you receive will range from parts of a coin to several whole coins. You will see the duplicate cards fill up the coin meter when you open your packs.  

Duplicate tiles: Since the tiles you get when using your collection coins are random, it's possible that you will win a tile that you already own. When this happens, you will receive Scrabble Coins. These coins can then be used to get any tile in the game or any past collection that you don't already own. This makes them very valuable!