If you’ve got specific accounts you want to follow, you can get an RSS feed containing their public posts.

In terms of native clients, the closest such feature I know of, is that Fedilab (for Android) can remember your position in the timeline, so that you can resume reading.
If you primarily use one timeline, then you wouldn’t re-encounter already read posts, because they’re in the past from the remembered position.
I have to say, though, that it’s not the most reliable feature in the world…

Boost for Lemmy un-bolds* the old ones