src/src.js
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()).