get your object and move it to the radius of the circle you want. Then hit insert to move the pivot point to the origin (Hint push and hold x to snap to the grid).
Then goto the duplicate options box and work out what increment you need (e.g. if you want 10 copies then 360/10 = 36. So you will rotate 36 degrees in Y and put the number of copies to 9 (because you already have one copy sitting there). And that's it done.
Technical Director - Framestore
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