How to Grouptext on Samsung Galaxy S4?

[AC Question]

I recently took my dad's old Samsung Galaxy S4. Whenever I try to start a group chat, it will send but then when people reply it opens up a separate thread, where my number is included. And then after I send things in that thread, the message I sent shows up again as "Me", like I sent the same message to myself in the group chat. My dad says that this was never a problem for him before, and that group texting showed up normally for him, but it must have reset when he gave it to me. Has anyone had this problem? How can I switch it back to regular group texting? I have tried looking in the settings, making sure it was switched on group messaging and not individual messages, etc.