In believe he was referring to the night before when they were both drinking and the teacher said he forgot half of the night but you can tell from his blush that he remembered the part where they confessed to each other. So by saying that he's going to tell him a second time, I think it means he is going to tell him again while sober what he had told him the night before. To show it wasn't just drunken talk.
SPOILERS
For the ending, was that the black haired dude, saying "I want to tell you my proposal even if it's the second time" or something? And when he said, "I don't mind how many times you say it", what exactly was he referring to? Can someone explain the ending to me. Thank you!