As you said, sure that no one denied the gathering of a lot of people at the event of ghadir khumm. And what prophet (saww) has said was “man kunto maola fahaza aliyun maola.” In this statement he used maola twice. Once for himself and the other maola was for Imam Ali (as). If we say maola means successor, then in the translation, Prophet(saww) calls himself successor too? And if maola means master then in the translation, Imam ali(as) is the master and also Prophet (saww) is the master. Were not there be a noise where there are two masters at the same time?