You could say: Sorry I forgot you guys, you're not less important to me... kidding I think there is lots of possibilities and each one has its use depending on the case, I would write, for example: 1. FYI (for your information, when you just want the other people to be aware of the email, and probably there will be no more responses over it) 2. Sorry, I forgot some emails, just resending. (same case as previous) 3. Please, ignore the previous email which does not include everyone, consider this one for further response. (Here you include everyone, even the ones who alredy received the email, because you expect people to answer you, and when they do everyone gets the new email, considering that the ones answering use the "reply to all" option). I think I would do something like that...

"I've CC'd so and so in." If it is a business email, I wouldn't send it twice. I would send it only to the person I omitted in the first sending of the email, and attach a note; "I thought this would be of interest to you."