I love this game but have never been able to put much more than 30 hrs into it, because I find the game has way too much text. I'm the kind of player who likes to talk to every NPC but then I just get bogged down.

Are there any mods that at least indicate to you when a particular line of text has already been read, including from another NPC, as many NPCs have the same answers to certain questions, that would at least prevent me from clicking on certain questions that I've already had an answer to from another NPC. I doubt there are any mods which add dialogue to the game but that would also be great?

There's no mods indicating read lines, and unless OpenMW or Project Aedra authors can be persuaded to add such functionality into their remakes... *and* unless they ever make their projects into something workable, you're out of luck. What you can do is to make dialogs less monotonous by adding mods from LGNPC series.

That would actually be a good feature IMO. Just to have topics with already received answers appear a bit dim.

I don't see it as possible though, but what do I know about programming...

Lack of dialog filters has always been a pet peeve of mine. Sometimes I wish topics would just write to my journal and disappear out of the game. It always niggles at me when I get the exact same response from two different people on the same subject. Unfortunately, in order for topics to go away, I think you'd either have to have a journal entry or a global variable for each and every response. The would be time consuming, would make the journal virtually unusable, and would probably slow the game down considerably. Oh well.

I don't mind the repetition while talking with someone; I just wish it wouldn't add identical dialogue to the journal.

Honestly, I usually don't even bother clicking on the generic topics like rumors and little secret. I probably miss something here or there, but it seems to mostly be tutorial - So, having played MW for so many years, I don't bother. In my own mod, I attach a "nolore" script to every NPC, which removes the topics - in their place, I'll put unique topics and unique answers, if I feel there need to be any.

However, there is this mod, which I haven't tried:
http://www.elricm.com/nuke/html/modules.php?op=modload&name=Downloads&file=index&req=viewdownloaddetails&lid=1932

I've had that installed in my game for over a year now such that I can't tell you if it's made a difference or not. Perhaps load order also impacts its effectiveness. It's around 16 on my load order after using mlox. I click on the dialog topics because I use LGNPC mods and find small variations at times.