If you've added one or more numbered or named capturing groups to your regular expression then you can insert backreferences to those groups via Insert Token|Backreference. In the window that appears, click inside the capturing group to which you want to insert a backreference. RegexBuddy automatically inserts a named backreference when you select a named group and a numbered backreference when you select a numbered group. The backreference will match the same text as most recently matched by the capturing group.
For example, the regular expression \b(\w+)\s+\1\b matches a word that it captures into numbered group #1. It then attempts to match one or more spaces followed by backreference #1. The backreference attempts to match the exact same word as the capturing group just matches. Essentially, this regex matches doubled words.