Would it sound weird if I omit "in" or "at" here and say "fit in this party" or "fit at this party"? Why are "at" and "in" used together in the sentence?
Yes, it would sound weird if you were to omit "in" or "at" in this context. Although it may seem like "in" and "at" are prepositions and one can be omitted here, "at" is the only one being used as a preposition in this context because "fit in" is a phrasal verb. It would sound weird to omit the preposition "at" after "fit in" and it is better just to keep them together.
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