Do Protestants today repeat ancient heresies without knowing it?

Yes, and usually without knowing it. There's nothing new under the sun: all those heresies about Christ, the Holy Spirit, and the Trinity keep repeating themselves over time. Take Calvinism: I believe it's in Calvin where he says the Father pours his wrath out on Christ's humanity but not his divinity. If you say that, now you're into Nestorianism, Christ sometimes divine and sometimes human.

A man who came to my church from Protestantism heard a whole sermon on Arianism and said afterward, Father, I think I was an Arian before, because I thought Christ was created by the Father. At my old job Christian coworkers would pray strange, essentially non-Trinitarian prayers. I've seen so many Protestants try to explain the Trinity and just explain Modalism.

A popular preacher said, in the Old Testament God came as the Father, in the New Testament as Christ, and now as the Holy Spirit. That's Modalism. He's not a Trinitarian. Many Protestants are well-intentioned, sincere people, without a doubt. But without understanding the historicity of the church and the early heresies, you can't even see what's wrong with the doctrine today.

If your argument is that Christian doctrine is whatever I read out of the Bible and believe, you end up in heresies. Even the term Trinity is extra-biblical: the idea is in the Bible, but you have to argue to the Ecumenical Councils, which shows you the church was the authority. My intention is not to bag on Protestants. It is for people to become educated, so they know this was never part of Christianity.