msgid ""
"the method for function %s and signature %s is sealed and cannot be re-"
"defined"
msgstr "函数%s的方法和%s标记都被封闭了，因此无法将之重新定义"

可能是以下问题导致...
sealed         If TRUE, the method so defined cannot be redefined by another call to setMethod (although it can be removed and then re-assigned). Note that this argument is an extension to the definition of setMethod in the reference.
http://www.stat.ucl.ac.be/ISdidactique/Rhelp/library/methods/html/setMethod.html