# Created on savesnippets.com · https://savesnippets.com/1b2AqZYZxzCGAk
from typing import Callable, Hashable, Iterable, Iterator, TypeVar

T = TypeVar("T")

def unique_everseen[T](items: Iterable[T], key: Callable[[T], Hashable] | None = None) -> Iterator[T]:
    seen: set[Hashable] = set()
    for item in items:
        k = key(item) if key else item   # type: ignore[arg-type]
        if k not in seen:
            seen.add(k)
            yield item

list(unique_everseen([1, 2, 1, 3, 2, 4]))                 # [1, 2, 3, 4]

emails = ["Bob@X.com", "alice@x.com", "BOB@x.com"]
list(unique_everseen(emails, key=str.lower))              # ['Bob@X.com', 'alice@x.com']