This drove me crazy.
In the previous post I wired up a React Native client app to a GraphQL Rails server. My app records meeting duration, so I added the server-side models in 33-minutes-server@838200 and client-side mutations in 33-minutes-app@b79e65 and 33-minutes-app@f253e4.
When creating a second item I would encounter the following error: Encountered two children with the same key, <id>. Child keys must be unique; when two children share a key, only the first child will be used. This was odd, because I knew all IDs were unique. Turns out, this is a Relay bug, with a proposed fix in relay#2349 that is yet to be merged. This is fairly basic and is a good demonstration of how
immature cutting edge this ecosystem is.
You can work around this by supplying a unique ID into each GraphQL mutation, which is an optional field in the Relay spec.
In my app this change is 33-minutes-app@bf4a89.