I think gmail adds some heuristics on top of it, like if the same image was included in emails to multiple people.

At least that's what I remember from them announcing the feature. No idea about other providers, and I haven't tested the feature myself.

With a proper personalized tracking pixel, a simple deduplication won't catch it—the whole point is that each email's tracking pixel has a unique URL that lets them know that you opened the email.

It is, of course, very possible that Google has heuristics that can catch tracking pixels—in fact, I would go so far as to say that if they chose to, they 100% could, probably tomorrow. But given where Google makes its money, I would not in the least trust them to do that for me.