wallet – How to protect my private key from an unexpected new owner of the address that already belongs to me?
The mitigation is the absolutely massive address space. Other than that, there is no way to ‘protect’ or otherwise ‘claim’ a keypair/address as yours and yours alone. If someone (by some unfathomable coincidence) manages to generate the same address that your wallet did, then you will both be able to spend any funds locked to that address.
Thankfully, the chances of this happening are super duper incredibly small. You would expend far more energy (money) looking for an already-used address than you could ever be expected to make by taking the bitcoin you are hunting for.
Perhaps worth mentioning: the above holds true iff the private keys for the address in question are generated in a suitably random (high entropy) way. If you use a poor source of entropy, then it become much, much easier for someone to brute force search for your keys.