ChromeProvider crash on duplicacted image region

[pchote]

Testcase: Add a duplicate spectator entry under flags.
Expected behaviour: the second spectator definition overrides the first (this is what happens if the entire flags entry is duplicated, and what we do for traits etc).
Actual behaviour: crash with an unhelpful An item with the same key has already been added. Key: spectator message that doesn't say where it is duplicated.

The current behaviour causes unnecessary problems for modders, and I expect that this should be a one-line fix (changing a .Add call somewhere to an indexed assignment).

[pchote]

There isn't a simple fix here and IMO the problem isn't worth the overheads in trying to solve it. Removing from the milestone.