Can I add more grout on top of existing grout?

Hi All! Last year, I had tile floors laid in a bathroom and laundry room and it looks great...EXCEPT there are places where the grout was not left thick enough and some of the thinset is coming through. The guy who did the work was supposed to come back and take care of it but never did and I cannot afford to pay someone else. I have a good bit of the grout left over and am confident I was wondering if I can just do another layer of grout over what's existing to make it thicker or if I need to remove the existing grout before adding more. Thanks for your help!
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