If it were a bubble, we wouldnt be seeing so many people buying these cards for themselves it would just be investors buying and selling and buying again in an effort to keep searching out the greater fool
Duelists can only have up to 1 copy in total of each Limited 1 card across all of their Decks, and up to 2 copies in total of each Limited 2 card across all Decks
If your deck is focused on this, some amount of milling to power them up can also be reasonable
Having four copies of each Night Marcher is essential for any Night March list, and running a full suite of Professor Sycamore helps the deck maintain resources and continue to draw necessary cards when other options are taken away
Cards like Blacklance Paragon, Dusk Legion Zealot, Kitesail Freebooter, and Seekers' Squire all could be better than Yarok's Fenlurker, so feel free to experiment
It turns out that forcing opponents to resolve your ETB triggers can lead to some very fast and unpleasant kills