ここを参考に Mondo Rescue のインストールを試みたが，次のようなエラーがでた．

The GPG keys listed for the "rhel 6 x86_64 - mondorescue Vanilla Packages" repository are already installed but they are not correct for this package.
Check that the correct key URLs are configured for this repository.

対処方法

/etc/yum.repos.d/mondorescue.repo 内に記載される GPGkey のURLを書き換える．

//etc/yum.repos.d/mondorescue.repo

# 変更前
gpgkey=ftp://ftp.mondorescue.org//rhel/6/x86_64/mondorescue.pubkey

# 変更後
gpgkey=ftp://ftp.project-builder.org/test/centos/6/x86_64/pb.pubkey

インストールしてみる．

$ sudo yum install mondo -y

Downloading Packages:
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing : mondo-3.2.2-1.rhel6.x86_64                                   1/1 
  Verifying  : mondo-3.2.2-1.rhel6.x86_64                                   1/1 

Installed:
  mondo.x86_64 0:3.2.2-1.rhel6                                                  

Complete!

以上．