How come it wont let me add someones Email on Exchange 2007?

Are you trying to copy one user on another users email?
That error usually means that the smtp address you are attempting to add is already being used by another mailbox.

I will need to go one by one checking each users mail box. Do you know of a faster way to find this?

This question tells you how to create a list:
https://www.experts-exchange.com/questions/26557659/Exchange-2007-list-of-all-smtp-addresses.html?sfQueryTermInfo=1+10+2007+30+address+all+exchang+list+smtp

I checked all mail box email addresses and no other mail box has the email address added (Secondary). Is the only way to do this by disabling (Active Directory) the owner of the mail box and then adding the email address as a secondary to another mail box?

open a new message in outlook
type in the address
Hit Ctrl+K to resolve it.
double click on the user it resolves to

that should show you who has that address.

Control + K did not do anything. I have highlighted and did this various ways.

try step three (ldap query) here:
http://exchangepedia.com/2006/03/how-to-find-an-email-address-in-active-directory.html

Can every email address only be associated with 1 mail box. Because its only associated to the original user. I have added the email address to other mail boxes before and not had a problem. Unless I have to disable the user in Active Directory?