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#! /usr/bin/haskell
import Data.List
someAssertions :: [String]
someAssertions =
  ["Brexit promises will be kept!"
  ,"18 July 2024 is the 150th anniversary of ‘The Hunting of the Snark’."
  ,"Brexit promises will be kept!"
  ,"Brexit promises will be kept!"
  ,"1 + 1 = 2."
  ,"18 July 2024 is the 150th anniversary of ‘The Hunting of the Snark’."
  ,"18 July 2024 is the 150th anniversary of ‘The Hunting of the Snark’."
  ]
atLeastThrice :: [String] -> [String]
atLeastThrice assertions =
  [head grp | grp <-
    group $ sort assertions, length grp >= 3]

Loaded and executed in GHCi:

*Main> atLeastThrice someAssertions
["18 July 2024 is the 150th anniversary of ‘The Hunting of the Snark’.","Brexit promises will be kept!"]