src/vs/base/common/arrays.ts/uniqueFilter
export function uniqueFilter<T>(keyFn: (t: T) => string): (t: T) => boolean {
	const seen: { [key: string]: boolean; } = Object.create(null);

	return element => {
		const key = keyFn(element);

		if (seen[key]) {
			return false;
		}

		seen[key] = true;
		return true;
	};
}