• Node 18 should be enabled (Refer this and this for more info)

• There should not already be a function definition in Preloaded.
• There should be an expected output format specification in Description.
• Kata should not be completely trivial.

Oh, and

• shouldn't that be stringify instead of stringfy ?
• PascalCase is conventionally reserved for constructor functions. All other identifiers conventionally use camelCase.

brudolce, random tests were added, however I have a few suggestions :

• what was used is a template which is very general and allows you to write tests for anything as fast as you can write a generator, however for some kata like this it is over kill, you could likely write the test generator in a few lines of code, I would suggest you play around with rewriting it

• it isn't clear if the first letter could be repeated (if that is, the generator needs to be updated, and doing so would invalidate a number of solutions)

• I think you mean every word has ONLY one double letter, this is a minor english point