mk-duplicate-key-checker: 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 CentOS possible default full paths are:

/usr/bin/mk-duplicate-key-checker - from package: maatkit

You may install the required package with command yum install {package}

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