topdown returns conflict error if same key/value pair produced multiple times

[tsandall]

OPA 0.2.1 (commit b3f62a1, built at 2016-11-23T16:33:13Z)

Run 'help' to see a list of commands.

>
>
>
> p["a"] = 1 :- true
> p["a"] = 1 :- true
> p
error: evaluation error (code: 2): multiple values for data.repl.p: rules must produce exactly one value for object document keys: check rule definition(s): p

The conflict check should allow for duplicate key/value pairs.