All I have been doing is showing you examples and not giving you instructions on what to do that choice is up to you.
No, it has nothing to do with if you want to use the same email, I'm just telling you what would happen if you did use the same email and didn't chose the "I'm a registered user and my registration key has expired, I need a new one" option you would just get a copy of your current unexpired key.
Again no, if you use the "I'm a registered user and my registration key has expired, I need a new one" option avast doesn't check to see if a) the email address is the same and if you current key is still valid.
I'm not saying use a different email address, just showing you an example that if you did, it wouldn't matter if you have an existing valid key you would be sent a new one as there is no way to link the new email address to a previous registration were a different email was used.