In Connect you can hide read posts automatically and scrolling past posts counts as read. Is there an option to auto hide read posts? I hate seeing the same post twice.
If you enable the FAB, you can arrange your frequent actions so that "hide all read posts" is first. Then, when you long press the FAB, it will hide all read posts.
Settings shortcut: General > Show FAB
FAB action has to be set to "more actions" (or whatever it's called).
Alternatively, you can just set the action to "hide read", but that will remove your ability to use the FAB for other actions.
Set it to hide read posts on lemmy settings. You'll have to login the web browser or use another app that can change lemmy settings (jerboa and connect are the ones I know can change it), sync doesn't have this setting, at least for now.
If you hit the three dots at the top right a menu appears and there's an option for hide read posts. I haven't tried it yet though to see how it works.