Duplicates are usually more ideal, but diversifying is also good.
If you have, say 2 AR15's or 2 Glocks, or 2 Mosin Nagants, you can have duplicates but also have different calibers (not the M/N). Different barrel lenghts, gas systems, grips, stocks, sights, etc.
Now if you want to strictly diversify i.e. AR15 and Mini14, or Glock and M&P, you will not be able to share magazines, accesories, and even ammo which is a big downside IMO.