Usually an ingredient is listed twice like that because different amounts are used at different times in the process. In this case, the 2 cups are added to the batter of the actual cookie.
The 3/4 cup is for rolling the ball in before placing on the baking sheet.
I've found that people do this because so many read a recipe quickly...and if it simply said 2 3/4 cups sugar people would put the whole amount into the batter.
Of course another reason might be that the 3/4 cup can be either regular or coarse sugar. I'm not sure that using coarse sugar in the batter would produce a pleasing end result in the cookie itself.