Whenenver I configure my project to label the project on success, the build happens repeatedly. It seems that the adding of the tag causes a change to be detected by teamcity which initiates another build.
Using TC 6.5.6 (build 18130) with mercurial.
I have created a test project with the minimal settings and it seems to keep happening.
Found a work around:
Put '-:/.hgtags' into the 'Trigger rules' field of the trigger.