Q: Email address and iCloud issue

I am using Mail 9.2 in OS X El Capitan.  The problem is an undesired but persistently recurring sender/recipient email address that continuously requires me to change the recipient’s email address manually.  Here is how it goes:

I receive email from x which is send from the account x now exclusively uses.  Previously, x maintained two different email accounts and addresses - I deleted the now defunct email address of x from his info in Contacts on the PC as well as from Contacts as backed-up on iCloud.  The iCloud contact info does no longer contain the defunct email address, but Contacts on the PC maintains it although it is now greyed-out, listed under “other”, and I cannot edit/remove it.  Whenever x sends me an email from the email account he now uses, the old defunct email address is displayed as sender and if I don’t manually change to the proper recipient email address - that is if I use only “reply to sender” - the email is send to the defunct account.  "Defunct" refers to the email account x now no longer wishes to use. 

How can this matter be resolved?  (1) I like to fully delete the now defunct email address; (2) when I receive an email from x, I like it to show the proper sender address; and (3) when I send an email to x, I like it to display as recipient address the one that x now uses. 

Another issue: after I went to iCloud to edit the contact info of x, all my contacts in PC”s Contacts were doubled and stayed that way even after disengaging from iCloud, which required me to manually delete all doubles.  Why does this happen and how can it be avoided?