There are a few possible reasons for this:
1. The sender may have accidentally entered your phone number more than once in their contact list.
2. The sender may have a group chat that includes your phone number, and you are receiving each message that is sent to the group.
3. You may have signed up for a service that sends out duplicate messages (such as a weather alert service).
If you are receiving duplicate texts from a specific sender, you can ask them to check their contact list to see if your number is entered more than once. If you are receiving duplicate texts from multiple senders, you can check to see if you have signed up for any services that send out duplicate messages.