Double Coins Forever will double the coin rewards for every game you play. It will not double the coins you already own when purchasing it.
The normal coin rewards from a full game without any additional rewards are as follows:
- 1st: 30 coins
- 2nd: 15 coins
- 3rd: 10 coins
- 4th: 5 coins
Double Coins Forever will double these amounts. Note that coin rewards will be smaller by default if you're playing a game with fewer than 4 players.
You can also get double coins in other ways, like the treasure chest in the post-lobby or a limited time promotion (e.g. Double Coin Weekend). These events will stack with Double Coins Forever so you can get 4x or 8x the normal amount of coins.