function filterDuplicateFiles() {
	const lookup = new Map();

	return value=>{
		if (lookup.has(value)) return false;
		return lookup.set(value, true);
	}
}
function filterDuplicateFiles() {
	const lookup = new Map();

	return value=>{
		if (lookup.has(value)) return false;
		return lookup.set(value, true);
	}
}
function filterDuplicateFiles ( ) { const lookup = new Map ( ) ; return value => { if ( lookup . has ( value ) ) return false ; return lookup . set ( value , true ) ; } }
Filter-out any files that are already loaded.

@returns {Function}		Filter function to track duplicates (for use in Array.prototype.filter()).
Filter - out any files that are already loaded .