A couple of releases have spit out warnings from docker tag -f, and Docker 1.12 is finally removing it. This has caused me to have to update some scripts to figure out if they’re using a version of Docker where it’s required vs. one where it’s disallowed (see also Docker 1.12 breaks “docker tag -f” in scripts).
Since Docker 1.10, the short answer is that docker tag -f doesn’t do anything from docker tag without -f, and if you’re sure you have a new enough version of Docker, you can just remove the -f option.