Yeah, you're not "allowed" to use the same letter twice in the expression of scale degrees. So in the key of C#, the letter C is already taken, so you have to call the 7th degree B#, though it's the same pitch as C. And in the same key, you would call the 3rd degree E# (not F) because the F is taken for the F#.
C# D# E# F# G# A# B# is "correct."
C# D# F F# G# A# C would get you drawn and quartered back in the day.
If you do it the wrong way, the notes on the staff get cluttered with a bunch of accidentals, instead of referring nice and cleanly to the key signature. That's really more of a classical music thing, like I said, where everything had to be neat and orderly. Jazz is more like, whoa, far out, man! Let's change keys seven times in this one measure!
Don't tell my teacher. I'm supposed to learn the song first.
