I assume you mean "How do you ensure that there are NO duplication of bugs in Test Director?".
In that case, it can be accomplished 2 ways: 1) Searching, and 2) Training.
1) You can use the very limited search functionality with the Defects Module to look for similar bugs prior to submitting new ones. Check the Users Guide for more info on the Search functionality. Also, you may have your admin add additional User-defined fields (such as Category, Module, Version, etc.). This will allow the users to filter Defects based on specified criteria.
2) Number 1 above is useless without Training. You must ensure that your users KNOW to search/filter prior to submitting new defects.