Let L be the number of littering citations the warden issued.
He issued 2L citations for littering and off-leash dogs, and 2 * 2L = <<2*2=4>>4L citations for parking fines.
In all, he issued 2L + 4L = 6L = 24 citations.
Thus, he issued L = 24 / 6 = <<24/6=4>>4 citations for littering.
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