Thanks for reaching out Carolissosmiley,
It sounds like you're receiving blue iMessages and green SMS messages from the same contact. We can help.
When we talk about using iMessages, these are sent using a data connection only. They also require both the sender and recipient to be using an Apple device. On the other hand, you'll see a green SMS message if a data connection isn't present for everyone, or if any member of the conversation is messaging on a non-Apple device.
You can learn more here: What is the difference between iMessage and SMS/MMS?
Since it's only that one contact, the issue must be on their end, OR you are choosing the different threads and SMS message when you reply.
Have them verify their settings: If you can’t turn on or sign in to iMessage or FaceTime - Apple Support
You can go through and delete the entire thread of the SMS, and only keep the iMessage thread, this should help from being confused and using the wrong one too.
Hope that helps.
Take care.