If qualifying word; such as this, that, next, every, last, all etc. comes just before some ‘Time denoting words’, e.g., morning, evening, day, night, month, year etc.; No preposition is used with them.
Example: He is coming / again on / next Sunday. / No error
Explanation: As per the rule given above, there is an error in the second part of the sentence; ‘on’ should not be used before time denoting expression next Sunday. So, the correct sentence should be:
He is coming again next Sunday.