- repository-pattern and repository
- factory-pattern and factory
- strategy-pattern and strategy
- observer-pattern and observer
- visitor-pattern and visitor
- command-pattern and command (latter not always the command pattern, but Thinking About Using Command Design Pattern is, so it's an ambiguous tag that needs attention anyway)
- specification-pattern and specification (latter not always about the pattern, but Specification Pattern and Performance is, so it's an ambiguous tag that needs attention anyway)
- facade-pattern and facade
- builder-pattern and builder
- and lots more, including null-object-pattern and null-object
Though there are tags where there isn't duplication, because it isn't a design pattern, such as url-pattern.
I asked about observer-pattern versus observer in Difference between the observer and observer-pattern tags? .