Creation of an object with a unique index (e.g. a keyed-object) failed. Even so a previous existent object with the identical key has been deleted in recent data, the object is still visible in the consistent view of the recent transaction. The newly created object would cover the still visible object and therefore the creation is cancelled.
Make sure that the same object is not being deleted or recreated in different transactions of parallel sessions. Create a new consistent view after the error occurs (with omsNewConsistentView) and then create the object.