Remove first encountered elements from a second list
APL(Dyalog Unicode), 8 6 bytes SBCS
/⍨∘~∘≠

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A function submission which takes L1 on the left and L2 on the right.
-2 bytes from Bubbler.
Explanation
/⍨∘~∘≠
/⍨     remove elements from L! identified by:
   ~   the bitwise negation of
     ≠ unique mask of L2