I've got records from patients with different cancers. Naturally, common cancers were registered often, and the cases in this group are then overrepresented and vice versa for the rare cancers.

Let's say that I have:

 - 20 patients cancer 1  
 - 160 patients cancer 2 
 - 80 patients cancer 3

-> are these strata uneven?

I would like to iteratively sample a set with an equal amount of sufficient cases from each cancer to test on.

However, if I pick 20 cases from each cancer, this won't cover cancer 2, and I will include the same patients from cancer 1 in each iteration. 
Is this a problem?
If so, how would I solve this problem? 

If I use sampling with replacement, I do not necessarily pick the same 20 cases from cancer 1, but wouldn't I need more iterations ensure cases from the larger strata are covered enough?