If you are seeing two message threads on your iPhone, there are a few possible reasons:
1. You may have duplicate contacts saved on your phone. If you have a contact saved under two different names or with two different phone numbers, you may see two message threads in your Messages app. To resolve this, you can try merging the duplicate contacts.
2. You may be using multiple messaging apps on your iPhone. If you have installed multiple messaging apps, such as WhatsApp or Telegram, you may see separate message threads for each app. To reduce clutter, you can consider deleting the messaging apps you no longer use.
3. You may have enabled Messages in iCloud. If you have enabled Messages in iCloud on your iPhone, it may sync your message threads across multiple devices. This could explain why you are seeing multiple threads on your iPhone. To disable Messages in iCloud, go to Settings > [your name] > iCloud > Messages and toggle off the switch.
In conclusion, seeing two message threads on your iPhone can be caused by duplicate contacts, multiple messaging apps, or Messages in iCloud. You can resolve this by merging duplicate contacts, deleting unused messaging apps, or disabling Messages in iCloud.