All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ʻalayhi wa sallam, is His slave and Messenger.
The scholars held different opinions regarding the ruling on reciting the Faatihah by the one led in prayer. Ash-Shaafiʻi held that it is obligatory on the person led in prayer to recite the Faatihah in all prayers, whether those where the imaam recites aloud or silently. This is the view held as preponderant by Shaykh Ibn Baaz and Shaykh Ibn ʻUthaymeen and it is the view which we adopt at Islamweb. Accordingly, if the person led in prayer finished reciting the Faatihah before his imaam, he says Aameen silently and then says Aameen again when his imaam finishes reciting it. Based upon this, you did what is prescribed for you to do according to the view adopted at Islamweb.
Your action has no effect on the validity of the prayer in any case, whether according to the scholars who held that the one who is led is obliged to recite the Faatihah or those who held that he is not obliged to recite it.
Allah knows best.