I'm using p7zip 9.20.1 and if I'm using -mhe=on on non-empty files and the password is empty p7zip asks on decompression 2 times for the password. Steps to reproduce:
1. Create the file test.txt with the content "0".
2. Use "7z a -mhe=on -p test.7z test.txt"
3. Just press enter on the password prompts.
4. Use "7z x test.7z"
5. p7zip asks now 2 times for the password.
This ticket should maybe depend on https://sourceforge.net/tracker/?func=detail&aid=3544566&group_id=111810&atid=660493 because it will maybe automatically fix this problem.