Signing from your wallet with an address that belongs to you. Which wallet are you using?
If someone else is actually really using your wallet already, then you'd best ditch your wallet and sweep all your funds out.
Huh.. But we can both signing from my wallet...
*It's not a real situation! It's just a model!Pretty much the answer is: if you hold the keys, they are your coins. So if you both have the keys, you better move the coins immediately to a wallet that you control. And if a criminal has the keys, they probably will have already done that.