A third issue is the likelihood that the same materials be registered more than once. This can happen when projects that operate independently and are not aware of each other, continue to conduct research on materials from a completed project. However, this problem is common to other identifier systems, and the equivalence of identifiers and how to handle it, has been studied . A practical solution to this problem is to simply use an identifier mapping showing that two identifiers refer to exactly the same material. It is important to realize that this approach is a lot more tractable than accidentally using the same ERM identifier, and then later realizing that the materials are not identical. Indeed, there are plenty of solutions to handle the equivalence of identifiers, like the ELIXIR Recommended Interoperability Resource BridgeDb .