Burntsushi has written many important crates in the Rust ecosystem. He started with licensing under Unlicense exclusively, until people requested a dual license with MIT. See this issue from 2016 for more details - https://github.com/BurntSushi/byteorder/issues/26

Almost all of the Rust ecosystem is dual licensed under MIT/Apache 2.0, so this combination is a bit unusual. But the presence of MIT means that it hasn’t been a problem in practice.