  The spin is an important property of a particle. Although it is unlikely to
happen, there is still a possibility that two particle with different spins
share similar masses. In this paper, we propose a method to probe this kind of
mass degeneracy of particles with different spins. We will use the cascade
decay $B^+\to X(3872)K^+$, $X(3872)\to D^+D^-$ to explain our method. It can be
seen that the possible mass degeneracy of $X(3872)$ can lead interesting
behavior in the corresponding cascade decay.
