I created Ropsten testnet account through Metamask. But in Etherscan, it seems that it is already used account, because it has some already made transactions. Why this happens?
In theory there are multiple private keys that generate the same address. So it is possible that your private keys generates and existing address.
But in practice this should be a very rare event.
Make sure you really have a conflict, I think you are very lucky in such case. And probably you want to generate a new private key.
Very funny. This is a very rare case. If that account contains hundreds of ether, you can stole them and nobody can say nothing. Of course this can happen for your ethers too by the other user who opened the account before than you! I’m very curious about this: in theory it cannot happen in many lifes. Tell us some details: it is very interesting to understand it better.