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!Anyone who has the private key own the wallet. So if it happens that someone got your private key, then that person also owns that wallet. We can identify the true owner by the record that has the earliest record or file of wallet.dat or json file if it is in electrum. If it is created online, you can check the history of each pc as long as the cookies or history of the browser is not deleted. If all is not available then we cannot prove the true owner of that wallet.
That at least what I think to verify the true owner of that wallet.
I also agree with buwaytress to forget about the wallet if ever that is already emptied out by the owner or the hacker and never use it again because it is already compromised.