lp-set-dup: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/lp-set-dup - from package: lptools

You may install the required package with command apt-get install {package}

Error in other languages:
lp-set-dup: bevel nie gevind nie
lp-set-dup: comandă negăsită
lp-set-dup: comando não encontrado
lp-set-dup: commande introuvable
lp-set-dup: command not found
lp-set-dup: käsku ei ole
lp-set-dup: không tìm thấy lệnh
lp-set-dup: komanda nerasta
lp-set-dup: Kommando nicht gefunden.
lp-set-dup: kommandot finns inte
lp-set-dup: komut yok
lp-set-dup: nie znaleziono polecenia
lp-set-dup: níor aimsíodh an t-ordú
lp-set-dup: no se encontró la orden
lp-set-dup: no s'ha trobat l'ordre
lp-set-dup: opdracht niet gevonden
lp-set-dup: parancs nem található
lp-set-dup: perintah tidak ditemukan
lp-set-dup: príkaz nenájdený
lp-set-dup: příkaz nenalezen
lp-set-dup: команда не найдена
lp-set-dup: командата не е открита
lp-set-dup: 命令找不到
lp-set-dup: コマンドが見つかりません