This page in a nutshell: It is usual Wikipedia practice that islands and administrative units that are largely coterminous have a single article.
Some people think that islands (as a geographical/geophysical concept) and administrative units (such as provinces, states, and municipalities) should have separate sets of articles, even if an island and an administrative unit are more or less coterminous. For example, these people believe that Greenland, the world's largest island, should have a separate article from Greenland, the autonomous country in the Kingdom of Denmark.
But current Wikipedia practice is that if an island and an administrative unit are more or less coterminous, then the two concepts should only have a single article.
This is easy to see why. When you have an article about an island, the article has to be comprehensive and not only discuss the island as a physical place. The article has to also describe the island as a place of human habitation and use, including current and historical information about human activity on the island. In the same way, an article about an administrative entity also describes the geography and lay of the land that the unit encompasses. If you try to meet both goals with separate articles, then you end up with two largely duplicate articles, which is obviously not the ideal case.
To elaborate on the point that islands articles need to be comprehensive, the following Featured Articles about islands demonstrate that the article should also talk about the current and historical human activity on the island:
The idea that we only need a single article can be easily seen with numerous examples. The following is a non-exhaustive list of articles about islands and their corresponding administrative units:
Anglesey, also a Welsh principal area called "Isle of Anglesey" containing other islands
