Why can't I have the same nickname for multiple Form Questions?

dup nickname

With the addition of sub-questions, the nicknames on those sub-questions are used to distinguish the data when importing or exporting the data. Since the nicknames are what are used for the column headers for the data, if they were all the same it would very difficult to ensure that the data, dates in the case, are attributed to the correct fields in the user's profile. 

As long as the text string is slightly different for each, it can be saved. You could even go the route of adding a number after each one to give some differentiation and save them.