It is possible to create Graphs with the same name using API, when several simultaneously running processes are trying to create Graph with the same name.

The other possibility (which I've seen first):
1) Make graph.create() with many graphs — request is quickly validated at once, but then objects are slowly (I've caught 10 sec socket timeout) created one by one.
2) Make another graph.create() with the same graph['name'].
If graph['name'] from the first request was not created by the time of the second request — we'll have two validated requests with the same graph['name'] and so two graphs created.