Here we need more information for the key generation, cause filemtime causes in error if you upload the files on the same time!
better here and then it works perfect is:
public static function key($ocPath) {
		$key = self::VERSION;
		$localPath = \OC_Filesystem::getLocalFile($ocPath);
		$aFileInfo=pathinfo($localPath);
		$nameOfFile=$aFileInfo['filename'];
		$key = $key. filemtime($localPath);
		for ($i = 1; $i < func_num_args(); $i++) {
			$key = $key . func_get_arg($i);
		}
		return $nameOfFile.'-'.md5($key).'.'.$aFileInfo['extension'];
	}