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"[*kommen*](https://en.wiktionary.org/wiki/kommen#Swedish)" is the past participle (*perfekt particip*) of [*komma*](https://en.wiktionary.org/wiki/komma#Swedish). From [this site](http://blog.lardigsvenska.com/2010/10/perfekt-particip.html),

> Perfekt particip is used as adjective and declines almost the same way as adjectiv. [sic]

An example given in the site of the past participle is "*maten är köpt*", with a translation being, "the food is bought", and "*den köpta maten ligger i kylskåpet*" for "the bought food lies in the cupboard".

I'm not a native speaker, but searching for both forms of "summer has come" yields results, suggesting that maybe they are both correct.