Is it possible to easily reduce 0/1 subset sum to subset sum with multiplicities?
This answer on CSTheory describes a reduction from 0/1 subset-sum to the unbounded knapsack problem (UKP).  That does exactly what you want.  The intuition is that UKP is basically subset sum with multiplicities, since UKP lets you put as many copies of an item as you want into your knapsack.