Determine behavior of repeated map keys

[EpsilonPrime]

When a map contains the same key multiple times there are a few options for the potential behavior:

1. Always choose the last value for that map key (as if the value had been overwritten by each occurrence).
2. Always choose the first value for that map key.
3. Return the full list of values.

One additional complication is the ordering that defines first/last.

This affects the documentation added in #521.