I don't like the fact that there's a mathematical possibility of getting the same challenge tokens again and again and again, especially for unpopular challenges..

It should remain random as to which come when, but it shouldn't be random how many of each are doled out.
An example;
1-2-3-4-5-6-7-8-9-10-11-12 Are all different challenges that we'll now shuffle into 12 days.
11-3-4-8-12-1-2-9-7-5-2-6-10 Is the order they will be given out for the given time period.

This will-
1: Distribute even numbers of tokens for all challenges.
2: Reduce the amount of 'dice rolls' done for free tokens.