I'm a little bit confused by your question, but I'll try to provide a few answers:
- If you are asking why the TeamCity server has a copy of your unit tests binaries at all, than you'll want to take a look at your artifact publishing configuration on the first page of your build configuration because it'll be configured to publish them, which isn't a bad thing unless you don't want them.
- If you are wondering why there are so many copies of the artifacts on the build server, than either you are just seeing one directory for each build stored on disk with a directory named with the internal build number.
- ... or your build script is creating a new directory on the build agent each time it runs and all the directories from previous builds are getting published again under the new build's artifacts, take a look at the "Clean all files before build" option on the VCS tab of the build configuration to ensure the build agent stays clean.