fdupes: 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/fdupes - from package: fdupes

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

On Debian possible default full paths are:

/usr/bin/fdupes - from package: fdupes

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

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