If the thing you "put" into is a CRDT, then two concurrent "put" will be merged, if that's OK for your application. If however you want to disallow concurrent "put"s then you need to add some concurrency control to the CRDT layer. See the CISE tool https://youtu.be/HJjWqNDh-GA, http://dx.doi.org/10.1145/2911151.2911160. Antidote doesn't yet support concurrency control, it's work in progress.