Allow identical enumeration values

[Matthias Simon]

We should allow identical enumeration values.

For technical reasons is not supported, yet:

ntt -i
ntt dev (none, unknown)
>> type enumerated U { E }
>> type enumerated V { E }
Error: can't add key E, name aleady exists as U.E

Originally posted by @5nord in #637 (review)

[Dawid Urbański]

#685