Very good question. I'll take a shot at breaking this down for you. Generally...chords consist of a root, a 3rd and 5th. You have identified all of these correctly. For a Bb chord they are Bb, D and F respectively. The A is the seventh and therefore you have 4 tones to complete this chord. Theoretically you are correct that the 2 F's are part of the chord sequence, but I would avoid both of the F's on the 1st and 6th strings due to tonality. The lower F (6th string) is a lower tone than the root note (Bb) and therefore would change the dynamic of what you're trying to accomplish.......which is the Bb7 chord. The upper F (1st string) would create some dissonance since you already have 4 tones in the chord and the F on the 4th string fits with the other 3 tones (notes) the best. Good rule of thumb is try not to play more notes than you have to. Hope this helps!