code: function filterDuplicateFiles() {
  	const lookup = new Map();
  
  	return value=>{
  		if (lookup.has(value)) return false;
  		return lookup.set(value, true);
  	}
  }
docs: Filter-out any files that are already loaded.
  
  @returns {Function}		Filter function to track duplicates (for use in Array.prototype.filter()).