Show warning on duplicate struct keys

[kubukoz]

Currently, duplicate keys in struct members are ignored (the choice is arbitrary, I guess the first value will be used for typechecking and so on).

It should be implemented in a consistent way (I'm thinking the last value should be used, which would allow for some possible destructuring operators in the future and so on), and a warning should be emitted on unused values (with the ability to remove them).