I repeatedly had this problem where the sketch file would be open in two tabs. This is in Arduino IDE version 2.0.3
I suspect the problem is the sketch file was stored in a file path that was about 165 chars long. The IDE may not allow for path strings to be that long.
This can be a problem if you are using the ELEGO folders to store your sketch file (because the paths are long).
I moved the sketch folder for this sketch file to another location, and the file now opens up in a single tab.