In any given key, there will be only one letter A-G whether it is natural, sharp or flat. E.g. if the key you are in has a "Bb", then there will not be any other "B" (sharp, flat or natural) in that key. However, that doesn't mean that the composer won't write a chord that contains another type of "B". But, that "B" won't be in the given key.
For example, in the key of "C" there is a "B" but no "Bb". However many songs in the key of "C" contain "C7" chords, in which the "7" of the chord is a "Bb".
Get it? No? Don't worry, you will get it.