Suggested you keep scripts that are intended to run once on player join in places like StarterPlayerScript or inside a ScreenGui with the ScreenGui ‘ResetOnRespawn’ turned off so it won’t run again.
Where is your code that adds the buttons? You might have put in StarterCharacterScripts, so it runs every time a character is added. If you did put it in StarterCharacterScripts, try moving it to StarterPlayerScripts