By the way, this post is yet again a good example of Vixen's need to denigrate Sweden and the Swedish.
The name "Utö" is Swedish and it means simply "Outer Island". There are at least two Utös in Finland (and I strongly suspect there are more). I suspect that there are several in Sweden in addition to the most famous one next to Stockholm but haven't checked. They are all place names and there's absolutely no reason to consider any of them "the real one". They all are real places with that name. The Finnish island has a Swedish name because it is in an area that has been populated by Swedish-speaking people since before the start of recorded history. (Which starts quite late in Finland).
The documented history of the more famous Finnish Utö starts at 1540s. There are claims that a pile of rocks was built there as a sea sign already in the 9th century, but I personally find that claim a bit dubious. I didn't go digging through the sources to see who proposed that, when, why, and based on what evidence. Quite a few internet sites mention it as an established fact. It may be true, but I need some convincing before I accept that.
On the other hand, the Utö next to Stockholm has documented history going to several hundreds of years earlier. It is mentioned by that name in the Danish Itinerary that was written in the 13th century (probably sometime between 1240-90). The itinerary doesn't mention the Finnish Utö and we shouldn't really expect it to mention it because the sailing route that it describes goes well North of the island, from Kökar to Aspö.
The Stockholm Utö was mentioned earlier than the Finnish one, it got permanent population before the Finnish one, and it now has far more inhabitants than the Finnish Utö. I don't see any reason why it would be less "real" than the Finnish one.