Haskell Twitch (high level file watcher DSL) executing 2 times

Question

I want to use Twitch package to copy any javascript files in the "central" directory to the "back" and "front" directories. This is the code:

{-# LANGUAGE OverloadedStrings #-}


module Main where

import Data.List
  ( isPrefixOf
  , tails
  , findIndex
  )
import System.Directory 
  ( createDirectoryIfMissing
  , removeDirectoryRecursive
  , copyFile
  )
import System.Directory.Recursive 
  ( getSubdirsRecursive
  , getFilesRecursive
  )
import Twitch
  ( defaultMain
  , (|>)
  )


main :: IO ()
main = do

  putStrLn "Haskell works <(^u^)>"

  -- Copy central folder to back and front, and setup recopy when central changes
  copyCentral
  defaultMain $ do 
    "./central/**/*.js" |> copyFileToBackAndFront




copyCentral :: IO ()
copyCentral = do

  createDirectoryIfMissing False "./back/src/central/" 
  removeDirectoryRecursive "./back/src/central"
  createDirectoryIfMissing False "./front/src/central/" 
  removeDirectoryRecursive "./front/src/central"

  centralFiles <- getFilesRecursive "./central/"
  centralDirs  <- getSubdirsRecursive "./central/"

  mapM_ (\d -> createDirectoryIfMissing True $ "./back/src" ++ tail d) centralDirs
  mapM_ (\d -> createDirectoryIfMissing True $ "./front/src" ++ tail d) centralDirs
  mapM_ (\f -> copyFile f $ "./back/src" ++ tail f) centralFiles
  mapM_ (\f -> copyFile f $ "./front/src" ++ tail f) centralFiles


copyFileToBackAndFront :: FilePath -> IO ()
copyFileToBackAndFront absolutePath = 
    maybe 
      (putStrLn "Error in the file path")
      getRelativePathAndCopy
      (findIndex (isPrefixOf "central") (tails absolutePath))
  where
    getRelativePathAndCopy n = do
      let relativePath = drop n absolutePath
      copyFile relativePath $ "./back/src/"  ++ relativePath
      copyFile relativePath $ "./front/src/" ++ relativePath
      putStrLn $ "Copied file " ++ relativePath ++ " to back and front"

And surprisingly it works! But i get the putStrLn output 2 times:

Copied file central\models\Board\Board.js to back and front
Copied file central\models\Board\Board.js to back and front

And I suspect it may be because of the program running in 2 threads, because sometimes I get interlaced output:

CCooppiieedd  ffiillee  cceennttrraall\\mmooddeellss\\BBooaarrdd\\BBooaarrdd..jjss  ttoo  bbaacckk  aanndd  ffrroonntt

CopiCeodp ifeidl ef icleen tcreanlt\rmaold\emlosd\eBlosa\rBdo\aBroda\rBdo.ajrsd .tjos  btaoc kb aacnkd  afnrdo nftr
ont

Also this happends when i run cabal v2-repl, i just tried cabal v2-run myprogram & and it does not work at all :(

Any help with both issues please?