Is this your script or part of your .bashrc or .profile?
First, if it's a part of your .bashrc or .profile, then remove #!/bin/sh.
Second, ip command can be run by superuser only, that's why you're getting error of 'command not found' when it's run with normal user. So you have an option to use sudo before it.
Code:
# Display IP address
sudo ip addr show eth0
Third, I'd suggest that you should remove this part from your .bashrc or .profile, because it will need root privilage and you will have to supply password everytime.
Code:
# Display IP address
ip addr show eth0
And if your script is running twice, then possibly you have added this script to some common shell initialization file like /etc/profile etc, therefore it's running for every user i.e. cntuser as well as root.