This is a one character spelling error that should be super easy fix.
Careers saves the previous cover letter and brings it up when I apply to another job. It also throws a useful warning:
This cover letter is identical to another that you send.
present-tense "send" should actually be past-tense "sent".
...that you HAVE SENT...
, but i'd ask the english.stackexchange.com people to chime in..