At the start or end of some levels where there are a lot of chests or diamonds, you can go back or forward through the door and collect the items again. In some cases you may need the triple jump to access these areas.

An example at the end of level 1.3, start of 1.4:


Another at the end of 1.4, start of 1.5:


This is an instance somewhere on chapter 3 (sorry, forgot what level!). The level starts like this:


If you go back again, (must be armed with the rocket rocket launcher to remove the grey blocks) you can collect the chests again: