RTR: Dedupe items in getKnownTasks.

[gianm]

Fixes a problem where the tasks API in OverlordResource would complain about duplicate keys in the map it's building.

[kfaraz]

Thanks for the fix!
So I guess there would be duplicates if say a task moves from "PENDING" to "RUNNING" and the running set is updated but the the pending set is still not updated?

[gianm]

So I guess there would be duplicates if say a task moves from "PENDING" to "RUNNING" and the running set is updated but the the pending set is still not updated?

The sets can be updated while getKnownTasks is running, so it's possible for the same task to be in both pendingTasks and runningTasks, if it moves in the time between reading those two sets.