To remove an outdated formaly used e-mail address from the list taken into account when Apple Mail does autocompletion of e-mail addressss, open the list of previous adressees from the “Windows” menu, select the address in question an remove it from the list.

Using different sender e-mail addresses (aka “aliases”) for one(!) e-mail account in Apple Mail is as easy as that: Just list all addresses in the “Address” field of the account configuration form separated by commas. That all …

References: Mac OS X: Using email aliases in Mail (by Apple, archived & unmaintained)

defaults write com.apple.mail AlwaysShowDuplicates -bool true

defaults write com.apple.mail _AlwaysShowDuplicates -bool true

References:
Apple Support Communities: How do I show duplicate email in Mail 6.0 (Mac OS X 10.8)
AskDifferent: How to get Mail.app to display (rather than hide) duplicate messages

OSX email