Both F and B are duplicated. This, of course, depends entirely on what you want to do. If you have a reasonable knowledge of music and you want to play Jazz or classical music, a chromatic harmonica is probably best. If you want to play blues or folk music or the likes, a diatonic is best. For blues you would not want to have a tremolo or octave tuned harmonica, because you can't get the juicy bends you need for blues on these.
Some keys are more useful to have than others.
If you are more apt to play along with horns, the flat keys come in handy, namely F, Bb, Eb or Ab. G. Some are available in Eb and other keys. Kinds Of Harps. Breaking In A Harmonica. Tuning Your Harmonica.