As I recall, last summer, it was various lengths for women's dresses at formal nights. I had been the "mother of the groom" the year before and had a nice ankle length outfit that took up very little room in the suitcase. I wore it for both formal nights on our back to back Med cruise and got compliments both times. I had no problem with repeating it at all! Who cares? I, for one, never remember what anyone wore! Chances are, you may not be with the same group everytime anyway. Buy what you will feel great in and have a ball.
Maggie