What's the point of cleaning the same spot again and again. It's not even dirty.
Why are you cleaning the same place over and over? I don't think it's dirty, is it?
ーWhat's the point of cleaning the same spot again and again? It's not even dirty.
「何のために同じ場所をなんども掃除してるの？汚れてもいないのに。」
What's the point...? で「何のため・何になるの？」という言い方。
again and again 「何度も」
ーWhy are you cleaning the same place over and over? I don't think it's dirty, is it?
「何で何度も同じところを掃除してるの？汚れていないでしょ？」
Why are you...? のように「なぜ…してるの？」と理由を聞いても良いですね。
over and over でも「何度も」
ご参考まで！
What are you cleaning the same spot over and over for? It's not getting dirty again.
Why are you vacuuming the same spot over and over? It's not getting dirty again.
Why are you cleaning the same spot over and over? Once is enough.
What are you cleaning the same spot over and over for?
何度も同じとこ掃除してどうすんの？
これはカジュアルな会話ですので「何度も」が "over and over" になります。「掃除する」は "clean" が良いですが掃除機ならもっときちんと "vacuum" と言います。「掃除機」は "vacuum cleaner"です。
「汚れていないでしょ」は "It's not getting dirty again." で意味を伝えられますがあまり自然ではないと思います。この状況で "Once is enough." つまり「一度で十分です」の方は良いと思います。
カナダ 
アイルランド 
