Using Alt to duplicate frame in the Timeline doesn't work all the time. Sometimes the little plus green icon appears and it works but most of the time it doesn't. I'm using the 3.0.91 version. My specs are iMac, 2,5GHz i5, 12 GB Ram, AMD Radeon HD 6750M 512MB Thanks!!!
Sorry for posting, I guess I was doing something wrong. I've been testing it and it works!