Simple reduction to unbounded knapsack?
There is a simple reduction from the subset sum problem. (We usually give it as an exercise.)

The idea is to encode in the weights that an element can only be included 0 or 1 times.

Assume the subset sum instance consists of numbers $w_1,\dots,w_n$ with target $W$. Assume that $w_i &lt; B$ for all $i$.

We will have two new elements for each old element, simulating whether the element is used 0 or 1 times. For element $i$ we get two new weights $w^1_i = (2^{n+1} + 2^i)nB + w_i$ and $w^0_i = (2^{n+1} + 2^i)nB$. The new weight bound is defined as $W' = (n2^{n+1} + 2^n + \dots + 2^1)nB + W$. Values of elements are the same as their weights, and the target value is $W'$