How to fix the issue "A feature with ID ... has already been installed in this farm. Use the force attribute to explicitly re-install the feature."

Use -force flag on your deploy operation. For example:

stsadm -o deploysolution -name MyBranding.wsp -immediate -force
stsadm -o execadmsvcjobs
stsadm -o activatefeature -name Mybranding -url http://testurl -force