Cucumber with Java and Selenium falsely reports duplicate step definitions

I've started getting this error when trying to run a cucumber test:

No tests were found
Exception in thread "main" cucumber.runtime.DuplicateStepDefinitionException: Duplicate step definitions in e2e.brukerveiledning.BrukerveiledningSteps.jegGårTilLivssituasjonForAaSeNyFunksjonFor(String) in file:/home/frank/Desktop/saksbehandling/e2e/cucumber/target/test-classes/ and e2e.brukerveiledning.BrukerveiledningSteps.jegGårTilLivssituasjonForAaSeNyFunksjonFor(String) in file:/home/frank/Desktop/saksbehandling/e2e/cucumber/target/test-classes/

Now, I really don't understand this error message. It claims there is a duplicate step definition, and it points to a folder, calling it a file. Also, it's an output folder, and not a folder containing actual source files. So how does anything get "duplicate"?

And this goes for a lots of steps and definitions that have been working fine up until now. Every time I comment out one alleged "duplicat", it goes on to claim that another step is a duplicate. And for every single one of them, a search yields one and only one result.

Any ideas?

And the step definition it claims is duplicated IS NOT DUPLICATED. A search for the name yields only one result.

For the example above:

@Når("^jeg går til livssituasjon for aa se ny funksjon for \"([^\"]*)\"$")
public void jegGårTilLivssituasjonForAaSeNyFunksjonFor(String sakTag) {
    long sakId = ScenarioState.getSakId(sakTag);
    navigationPage.gaTilPath("/sak/" + sakId + "/livssituasjon", false);
}

When searching for this step definition, only one match is found:

If I comment out this definition and try to run the test(s), it moves on to the next one with the same claim, yielding an identical result.

So there's clearly some deeper error somewhere, since the error claim is completely bogues (seemingly, at least).

Also, when the error first occurs (in Jenkins, and reproduced locally afterwards), some tests run fine and som don't. But after trying to run one of those that failed, all other tests suddenly fail too.