Security analysis on a verifiably encrypted signature scheme

Recently, a randomized verifiably encrypted signature scheme without random oracle is proposed by Yang et al. However, according to the security analysis, it is found that given two signatures on a message, the adversary can forge a new signature on the same message. Therefore, their scheme has not the propriety of undenibility, which means that their scheme is insecure. Because a verifiably encrypted signature scheme is mainly used to construct the optimistic fair exchange protocols, it should have the strong unforgerity.