Do you remember what it was? This is how I would spell that in BQN, and you could write something very similar in APL (you don't have shift, so you'd have to write 1 drop 0 cat swap instead)

    ((«¬∘∧⊢)' '=⊢)⊸/

This works by building a boolean mask of spaces, and converting it to a mask of 'is a space, preceded by a space', negates that, and replicates out by that inverted mask (i.e. is not a space preceded by a space):

Here's stepping through it with some input.

       (' '=⊢)  "this  is a  sentence with       many   spaces"
    ⟨ 0 0 0 0 1 1 0 0 1 0 1 1 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 1 1 1 1 1 0 0 0 0 1 1 1 0 0 0 0 0 0 ⟩
       («' '=⊢)  "this  is a  sentence with       many   spaces"
    ⟨ 0 0 0 1 1 0 0 1 0 1 1 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 1 1 1 1 1 0 0 0 0 1 1 1 0 0 0 0 0 0 0 ⟩
       ((«∧⊢)' '=⊢)  "this  is a  sentence with       many   spaces"
    ⟨ 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 ⟩
       ((«¬∘∧⊢)' '=⊢)  "this  is a  sentence with       many   spaces"
    ⟨ 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 1 0 0 1 1 1 1 1 1 1 ⟩
       ((«¬∘∧⊢)' '=⊢)⊸/  "this  is a  sentence with       many   spaces"
    "this is a sentence with many spaces"

▲ dzaima 2 hours ago | parent | next [-]

Some alternative spellings:

    (¬∘∧⟜«' '=⊢)⊸/
    (¬·«⊸∧' '=⊢)⊸/
    {¬«⊸∧' '=x}⊸/ # should have double-struck x here (U+1D569), but hn removes it

	▲	icen 2 hours ago \| parent [-]
		There's also the much less vector oriented spelling: `(1 ∾˜ " "⊸(¬∘⍷))⊸/` which removes the first space of each occurrence of double spacing

▲ tosh 2 hours ago | parent | prev [-]

in k:

  s:"this  is a  sentence with       many   spaces"
  
  s@&~0&':s=" "