The change you have noticed is not a bug. We are showing the best solutions in the Successful submissions section on each problem page. The repetition of username is due to the fact that user has submitted some of the best solutions and we are showing all of them without keeping constraint of one solution per user.
I think it is not really a good idea to do such a thing as it over populates the lists. You should just show the fastest solution for a particular user. If someone attempts posting a similar solution with some modifications in order to improve on timings you will end up showing all of them what purpose is it really solving. The ideal presence of the list on the right is to see who all have solved the problem and not who has solved the problem how many times.