two public keys with same passphrase insecure? | can two hashes be compared?
No.

The passphrase you use creates a symmetric key that is used to encrypt the private key. You're not uploading the private key to the server, at least let's hope you aren't.

But even if you are doing that, the password is salted and iterated in hashing, and that means that the visible encrypted private key is radically different even if the password is the same, because the same password produces two different symmetric keys.