The Shulchan Aruch (102) rules that one who finishes the Amidah may not step back from his place as one normally days upon completing the Amidah if somebody is praying the Amidah within four Amot (6-8 feet) behind him. This applies even if he stands more than four Amot in front of the person, but would come within four Amot by taking the traditional three steps back.
If a Chazan finishes the silent Amidah but cannot step back because somebody is praying the Amidah behind him, may he begin the repetition even without taking his three steps back?
In the interest of sparing the congregation the trouble of waiting for the repetition to begin, Halacha permits a Chazan in such a case to begin the repetition without first stepping back. Chacham Ovadia Yosef, in his work Halichot Olam (vol. 1, p. 149), writes that the Chazan would not have to step back after the repetition, either, as he may rely on the three steps he takes when reciting "Oseh Shalom" at the end of the "Kaddish Titkabal" which follows the repetition. Nevertheless, Chacham Ovadia's son, Chacham David Yosef, writes in his work Halacha Berura (vol. 5, p. 15) that it is preferable for the Chazan in such a case to step back immediately upon concluding the repetition.
Summary: One may not step back after the Amidah if he would then be within four Amot (6-8 feet) in front of somebody reciting the Amidah. If a Chazan finishes the silent Amidah and somebody is still praying behind him, he should start the repetition without stepping back. According to some opinions, he should take three steps back immediately upon concluding the repetition, whereas others allow him to rely on the steps he takes after "Kaddish Titkabal."
