Failed to create robot simulation interface loader: MultiLibraryClassLoader:

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Hallo. I want to launch two robot in gazebo. When i only launch one robot, there is no problem. but when i launch two robots, there is a problem "Failed to create robot simulation interface loader: MultiLibraryClassLoader: Could not create object of class type gazebo_ros_control::DefaultRobotHWSim as no factory exists for it. Make sure that the library exists and was explicitly loaded through MultiLibraryClassLoader::loadLibrary()". so i think this is perhaps that i have load two libgazebo_ros_control.so, is that right? How can i solve this problem?