I've performed some tests; in the cases where I'm prompted again to accept a hash, this is due to connecting to a different server on the same network. Since each server has its own unique certificate, if you connect to ten networks, each with ten servers, and each server has an invalid certifiate, you will need to accept 100 invalid certificates.

I have added an auto-accept invalid certificates to the next version of mIRC, although you would probably be compromising the security of your SSL connection if you use it.