Prevent duplicate subscriptions in file sys feed
Prevent duplicate subscriptions in file sys feed

We would create the entire enumerator stack and then go to add
the duplicate subscription to the collection and it would fail,
so now we catch this earlier, since with the file system feed
once we create the enumerator we schedule it, so in this case we
had a run-a-way enumerator with no subscription reference off in
the ether queueing data to nowhere... but that's fixed now!

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