Perfect discrimination of an orthogonal $2\otimes 2$ basis may require collective separable measurements on two copies

We consider an orthogonal basis of ${\mathbb{C}}^2\otimes {\mathbb{C}}^2$ with three entangled states and one product state. This basis is not perfectly distinguishable by LOCC or even by separable measurements when a single copy of the unknown state is available. We show that with two copies no adaptive separable protocol, which measures the copies one at a time, distinguishes the basis perfectly. We then show that a collective separable measurement on the two copies does. Whether two copies suffice for LOCC remains open.