Editing contact

I recently had an issue where we had two contact entries for the same person with slightly different email addresses (the company had changed their email). I deleted one and tried to edit the other, but was restricted by CC with an error message that said 'this email address is already in your contacts'). 

I searched for this email address in every way I could, but never found it. So I deleted the entry and re-entered all the information. The entry re-appeared with all the previous engagement activity as if it was there all along!

There must be a CC bug that loses some addresses in the system?