Are you playing on steemdice.net?
This is virtually impossible to happen there.
See https://github.com/pharesim/steemdice/blob/gh-pages/js/app.js#L201 and L2 for reference.

It is of course possible to send to the other account using another wallet, or you were on a phishing site. I have no idea who the steemdice account belongs to, but as it was created long before steemdice1 I would expect it to be a name grapper, not a phisher...
If you can rule out the possibility of a typo on your end I highly recommend changing your active key/password and make sure you're on the github pages version which steemdice.net redirects to in the future.