How to animate objects duplicating? I want to make a progess bar, but the sort that is discrete, I have my base square, now in next frame, I need another duplicate cube to pop into existence, next to it
Proper way would be to keyframe visibility on a bunch of duplicate objects or Dupligroups.

You get the benefit of instancing and reduced memory usage, you can also group multiple objects together and instance through an empty.

If you don't require such complexity, all objects are the same and in a single mesh, then you can get away with simply animating an Array modifier.