After collecting the loot, I returned to the guild. Then I showed the receptionist that I completed all the quests.
"Oh, I also defeated some desert scorpions and collected their poison gland. There should be another quest I didn't take. Can I complete the quest without taking it?" I asked as I showed the receptionist my loot.
"…Yes. So it means that you completed six quests you originally took this morning in just a few hours and another quest that you didn't take. Seven in total. The reward will be deposited to your card," she said.
I gave her my card and after a while she returned it back. Now the amount is more than before. Still, I'm already rich. I didn't need the reward after every mission. But if I refuse it, the guild's reputation would be worsened because people will think that the guild refused to give rewards to someone who completed a quest. That's why I still accepted the reward.
"…Will you take another quest after this?" she asked.