I replied to one of your emails. Can you check if you got it?
I think you accidentally sent me two emails, so I replied to one of them. Can you check if you got it?
I think you accidentally sent me two emails,
(あなたは多分私にメールを間違えて2通送ったと思う)、
so I replied to one of them. (なので、1通には返信したよ)。
Can you check if you got it?(確認できる？)
参考までに！
I've already replied to one of your emails, please check and see if you received it.
I've already replied to your first email, please check and see if you you received it.
「もう一通のメールに返信した」は英語で "I've already replied to one of your emails"
もう--> already
一通のメール--> one email,
この状況で"first email"も大丈夫です。
"please check and see if you received it"
「返信を届いたなのかを確認してください」みたいなことです。
アメリカ合衆国 
