On my server ~/.ssh/authorized_keys
contains only 1 public key. However, I can connect to it using 2 different private keys.
One private key looks like this:
-----BEGIN RSA PRIVATE KEY-----
MIIEoQIBAAKCAQEAqaO1eFD6XTTbmcItrptJxRJr89oW2gwlFU0tt8oF/6ZbOfV9
…
p1RjkISOfKl7YEElZEBHsl/ikCgv2C8DIOTTknXYDXeIxi/PMg==
-----END RSA PRIVATE KEY-----
The other private key looks like this:
-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAAAACmFlczI1Ni1jYmMAAAAGYmNyeXB0AAAAGAAAABDwMB0nEv
…
LlI3s=
-----END OPENSSH PRIVATE KEY-----
Why?