Check duplicate sample in a ServiceXSpec

[kyungeonchoi]

Story

A ServiceXSpec can have multiple Samples. The current version of client (v3.0.0) allows a user to have multiple samples with the identical dataset and query in a ServiceXSpec. It happily runs both requests (although the same query on the same dataset) and downloads both at different data directories. Only first request is cached.

Acceptance Criteria

ServiceX client should throw an error when there are samples with the identical dataset and query prior to submit to the server.