it's the concatination of the t and the y, so it would be "didn choo" or "don choo"A real annoyance - spoken only (hopefully)
"choo" instead of you, "didn't choo", "don't choo" etc.
There are accents where the difference between the two would barely be noticeable, especially some US accents.
My personal bugbear of late is unusual/inappropriate use of the word this, usually in place of "that" but sometimes just added on to the end of a sentence. Especially in the case of this/that it seems a daft difference, but to me there is definitely something wrong hearing it said out loud.
My girlfriend has a tendency to do it when she's overthinking what she's saying.