Mnesia {error,{application_name@host_name,{already_exists,application_name@host_name}}}

When you are going to create a mnesia schema with more than 1 node in nodes list you may end up with a error looks like following 

{error,{application_name@host_name,{already_exists,application_name@host_name}}}

Eg :-
{error,{yaws_app@ussdgw_02,{already_exists,yaws_app@ussdgw_02}}}

The cause for this error might be mnesia has been started in one or more nodes in which are listed in your nodes list in mnesia:create_schema([]). Stop mnesia by calling mnesia:stop() in all listed nodes.