At first I couldn't understand what you mean, now I do, but you're looking at it from the wrong direction.

The Ionian scale on C has no sharps or flats.

The same series of intervals, beginning with G, has one sharp, the F#.

The same series of intervals on D has two, the F# and the C#, etc.

The key signature is only a convenience, so that the sharps and flats don't have to be signed individually.

So if a piece is signed D (two sharps), it's true that C#, E and G are 'nothing', and that's why you are not likely to encounter that combination in that key.

Don't know if that's any help - I'm sure there'll be other replies.