Fruit of Luck stackable?

[Janus]

Title says it all, do i just get an ever improving bonus or does it use the highest bonus i got?

[Huillam]

It's an ever improving bonus.
If you're interested, tou can find the exact formula on the wiki: https://ngu-idle.fandom.com/wiki/Yggdrasil

[Fel]

More generally, all fruits outside of the first two are giving you permanent things (stats, AP, exp, macgufin levels or adding to that fruit's bonus stack).