I would provide an empty method body as a starting point. Now you're already providing half the solution and possibly pushing people towards a certain solution.

How could this have possibly worked? Are there no testcases for this kata?

There should have been an example of a passed test where there are two duplicate values next to eachother.

I would like to know what Im doing wrong. A test with an output like this only leads to frustration:

fullTest(EncodeTest)

null
Unknown error

Either show the testcode to the user, or show some meaningful text