Contrary to the replies in this thread, I'm pretty sure you can put "All modifications are dual-licensed under the GPL and MIT license" on your own original cod

Contrary to the replies in this thread, I'm pretty sure you can put "All modifications are dual-licensed under the GPL and MIT license" on your own original code and people can take and reuse the MIT licensed parts under that license - you can't relicense the already-GPLed parts though.