Does Subset Sum allow multisets?
One question we could ask is "Can we reduce this back to the subset sum problem?" In this case, the answer is yes: for each duplicate $z$ we replace it with two numbers $x$ and $y$ such that $x+y=z$. 
$$[-1,-1,2,3]\to [-7,-1,2,3,6]$$

However, we need to be careful that we don't introduce additional solutions (those using just $x$ without $y$), which we can do by making $x&gt;\lvert \Sigma(a_i)\rvert$ for $a_i&lt;0\in A$, and $y&lt;-\lvert\Sigma(a_i)\rvert$ for $a_i&gt;0 \in A$. Specifically, this precludes the use of $x$ without $y$ (and vice versa) by making the sum of $x$ and all negative numbers strictly above zero (and thus doesn't satisfy the traditional subset sum problem).