Pretty simple. I created a subclass and I want it to be able to learn any spell from any class in the game.
Problem is, when I add all the different classes spellbooks, it creates massive amounts of duplicates in the spell options, which is an eyesore.
Any fix?

Yes, don’t add the lists whole, add each additional spell individually so that there are no duplicates.

That's a LOT of spells haha.
Is there any way you can maybe copy paste a list in or something?

Also, it looks like the website simply crashes when I try and add more than like 30 spells...

I think there was someone a few months back who tried to add all spells to a character who got access via wish, and there was a limit. I think mods indicated it wasn't possible to up the limit enough to allow access to all. You could try adding the wizard class list, and then manually adding the rest, or adding wizard plus one other (cleric or druid probably has the least overlap) adding the rest manually, and just dealing with a smaller number of duplicates.

Yeah I'm currently trying to just go down the spell list and spam Enter so it adds them all individually, but I'm also running into the issue that its adding all Homebrew content and also anything from purchased source material, which is frustrating since I wanted to make the class Public eventually.

Is there a way to find a list of class EXCLUSIVE spells that are just part of the original spell list?

Try adding the spellbook of the class(es) with the most expansive list (probably wizard and cleric) and then just add the individual spells that are left from the other classes.

Exclusive spells:

sorc- chaos bolt
bard- vicious mockery, unearthly chorus, compulsion, power word heal
warlock- eldritch blast, arms of hadar, hex, hunger of hadar, shadow of moil
paladin- find steed, aura of purity, find greater steed, banashing smite
ranger- hail of thorns, zephyr strike, cordon of arrows, conjure barage, lightning arrow, conjure volley, swift quiver

And then there's druid that has like 20 exclusives. So you're not going to get it perfect if the site has a limit for adding individual spells.

Thank you! Going to try this, it seems FAR more effective at not crashing my page haha.

Oof okay so I just tried adding simply the Wizard and Cleric spellbooks to this Warlock class and I still have like 3 duplicates of half the spells in the options list =/

Again, you will have to add them individually, not as lists to completely avoid that.

This is the full list of spells that you can add that will still allow you to publish your subclass when adding them individually:

https://www.dndbeyond.com/spells?filter-class=0&filter-search=&filter-verbal=&filter-somatic=&filter-material=&filter-concentration=&filter-ritual=&filter-sub-class=&filter-source=1&filter-source=4

Any class from any spell in the game and 'all the spells available for free' are two quite different sets of parameters. Yeah, that'd have to be done individually.