What to do with duplicates?

[sobolevn]

For example:

@typeclass
def some(instance): ...

@some.instance(int):
def _some_int1(instance: int): ...

@some.instance(int):
def _some_int2(instance: int): ...

There are several things we can do:

1. Allow this, in this case we need to provide a test case and docs about when this can be useful
2. Add options, like @typeclass(final=True), it won't allow exact duplicates when activated, but by default we will allow it
3. Always forbid, but in this case people might end up with a situation when they need to override an existing behaviour but can't