Reported to the EvtGen dev mailing list:
Recently, I noticed that the decay B0 -> J/psi K0 (and its complex conjugate) appears to be double-counted in both DECAY.DEC and DECAY_2010.DEC, as it is once listed as simply B0 -> J/psi K0 and once as B0 -> J/psi K_S0 and B0 -> J/psi K_L0. I am using version 02.01.01. My apologies for not using the bug tracker. I am currently unable to access that webpage.