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
I have experienced this while making my topbar, if you are using a script that is placed in StarterGui I recommend you put that script inside a ScreenGui and set ResetOnSpawn in the ScreenGui to false and that should resolve the problem.