We report a study of the decay BDD using a data sample corresponding to 1.3 fb-1 of integrated luminosity collected by the D0 experiment in 2002–2006 during run II of the Fermilab Tevatron collider. One D meson was partially reconstructed in the decay Dsµ, and the other D meson was identified using the decay Ds where no attempt was made to distinguish Ds and D states. For the branching fraction Br(BDD) we obtain a 90% C.L. range [0.002,0.080] and central value 0.039(stat)(syst). This was subsequently used to make the most precise estimate of the width difference in the B- system: /s=0.079(stat)(syst).