After rendering this one video with subtitle, there are two subtitles.
Did you use shotcut to burn in the subtitles and embed them? If so, I would guess that your player is displaying the embedded subtitles - causing them to appear twice. Try opening the exported file in Shotcut. Does it show twice?
Use Export > Advanced > Other > Disable (embedded) subtitles checkbox if you choose to render the subtitles into the video within Shotcut. Not a bug