A few users have reported being tricked by other users into sending them Sweatcoins, which they then steal. You should be very certain that you can trust an unknown user before sending them even a small amount of your hard earned Sweatcoins.
Any user promising to double or triple your Sweatcoins is most likely a scammer. Even if you send them a small amount of Sweatcoins and they do double them, this is often part of the scam. As soon as you send a bigger amount they will keep them.
But always remember, there are no tricks or hacks that will allow you to earn more Sweatcoins - only physical movement and referring friends!