English is funny

One day recently, I received an e-mail reply that said

I resent it as well.

I was puzzled, and couldn't figure out why the person who sent me the e-mail was so mad. All that my original e-mail said was that I had sent an e-mail to someone else. Why would this person be resentful about my sending an e-mail that I was supposed to send...

...oh, never mind.