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彼は高齢を理由に社長を辞めたよ。

I found this example sentence in some core flashcards, and the translation says He gave up the company presidency due to age. Which makes sense to me, but I don't understand the purpose of the first を.

It seems to be indicating what the reason or cause for quitting is, but then why is 理由に there if を is already showing the reason?