Assume we are given $k$ bins of capacity $b$ and $n$ items with integral sizes $x_1,\dots,x_n$. The bin packing problem is to decide whether there exists an assignment of items to bins such that no bin exceeds its capacity. In this standard from, the bin packing problem is well known to be NP-hard. However, consider a version of the problem where every items has a twin of equal size. More formally, if $i$ and $j$ are twins, it holds that $x_i = x_j$.
For two bins, the problem is easy since there exists a feasible solution if and only if a packing where no twins share the same bin is feasible. However, what happens if we have more than two bins? I suspect that the problem becomes NP-hard. Unfortunately, I have not been able to prove this so far.
Does anyone have an idea for a reduction or know if the problem has been studied before? Also, if the problem is hard for twins, does the hardness carry over to triplets, quadruplets etc.?