Maps: are keys reliably unique?

The specification indicates that they should be treated as a dictionary / hash structure, but it isn't clear what should happen if duplicate keys are encountered. The specific scenarios are shared with https://github.com/antirez/RESP3/issues/35

and could be re-stated:

How is the following to be interpreted?

``` txt
%2
+ttl
:3600
+ttl
:9001
:3
````

is this:

- illegal
- valid, last value wins (so: `ttl` = `9001`)
- valid, all values should be preserved (so: `ttl` = `[3600, 9001]` in some way)
- something else?