Got it thanks. See now that avoiding duplicates only matters for output, not input; for instance aaabbbb would be valid input? ---> bababab... That was the point that was a little unclear to me, my mistake!

The y is a vowel in acidity, but a consonant in caditiy... Otherwise you would have a double consonant 'ty' or a double vowel 'iy' if y behaved as one or the other; it is contextual. This is what I was referring to.

acidity should return acidity as described in the example tests... Or else it should return failed if you conisder y a consonant in this case... What to do with y is an issue, doesn't fall neatly into either category.