These countries have more than one capital
https://edition.cnn.com/travel/article/countries-more-than-one-capital/index.html
When we see countries which have only one capital per one country, some of them are suffering from concentration of population and economy on one capital.
Japan and South Korea would be examples of the issue. Though Japan is equal to doing nothing about that, South Korea launched Sejong City for relocation of the ministries from the capital city of Seoul. But, judging from the increasing population of Seoul, it doesn't seem to be successful.
