Can I sand and re-varnish a flawed area on my staircase?
Of course you can do it, and you probably can even get the polyurethane to look pretty good. The trick might be how closely you can get the stain to match. If it's a very light stain or an extremely dark stain it will be more forgiving.

If it's a tone somewhere in the middle, you will need to judge your skill level, and make sure the color is spot on before re-applying the varnish.