There is already a thread exploring this same topic on the forum, do a search for it to get more points of view.
To answer your question, yes I have a back up of an iridescent charcoal caviar with ruthenium hardware in m/l. I believe the bag is from 15C (I may be mistaken). It is an exact same bag in same color and size and I ware one and keep the other one stored away. I personally do not regret having a back up, however I can see that this may seem redundant and wasteful to some people to have the exact same double. However, I know what gives me comfort and happiness and I do it even if it seems silly to someone else.
If you are someone who does not really like variety and want to ware one of the favorite peaces comfortably knowing you have a back up, than you will certainly not regret spending the money on backup. However, if you like a lot of variety and your peace may not get that much use in the long run, than perhaps investing in a different peace will work better.
Sorry for the long post. Hope it helps.