No, it is still a B♭ as the accidentals in the key signature and measure are never additive. The flat is just reminding you that the B is flat. This is known as a courtesy accidental and is typically done if the previous measure uses a B that was different then the one in the key signature or if there was a different quality of B used in the measure it is used to cancel out the other quality.
In the key D minor, if you were ascending from A to D, a typical melody would be A, B, C♯, D. If you were descending in the next measure back to A, your melody would be D, C, B♭, A. It would be typical to remind you the C is natural and B is flat.
In your example, the flat is to show you the 9th of the chord is flat even thought it is in the key signature. Also, the chord is wrong since a D(b9) has an F♯ in it instead of an F, so the chord is actually a Dm(b9).