when somebody send me BTCs I recognize that by seeing a transaction output is equal to my address. But how can I be sure that it's really me and only me who can spend the output?
I can check, I can unlock the output. But consider I'm an evil, I sell BTCs to somebody. But I create a multisign-script wich unblocks the transaction output also by my private key and when I receive $ I take my BTCs back.
Is there a protection against this? Do wallets check the output scripts somehow before claiming the balance?