I noticed you haven't gotten a response on your post just yet. If you could answer the following questions, it may help others in the Apple Support Communities get you to a solution:
- Are you seeing this as part of a group message? or
- Are you getting two threads for one person?
- What model iPhone and iOS version are you using (example: iPhone X, iOS 11.2.6. You can find this information under Settings > General > About).