Tagging friends on a workout is a Runkeeper feature that adds a workout to a friend's activity logs. This, in effect, makes it seem that the user's friend also conducted the same workout. According to Runkeeper:
Lets say you’re out for a run with a friend this weekend, and they’ve forgotten their phone. After you’ve completed your activity, you can tag your friends in it, not only so everyone can know that you guys are running buddies, but so that trip will show up on your friend’s profile! That’s not to say you can’t tag them if they remembered their phone! If they tracked as well, your run won’t override or duplicate on your friend’s profile.
For more information regarding this, you may reference this RunKeeper blog article: http://blog.runkeeper.com/1382/runkeeper-tagging-friends/
Please note that the Validic record for the activity logged by the user and the Validic record of the activity from the tagged user come as separate and unique individual workouts, mirroring the uniqueness of the records in Runkeeper. At this time, the records available from RunKeeper do not indicate whether an activity was the result of a tagging another user.