check for DUP argument as it is now ignored

git-svn-id: https://svn.r-project.org/R/trunk@66553 00db46b3-68df-0310-9c12-caf00c1e9a41
tweak

git-svn-id: https://svn.r-project.org/R/trunk@66554 00db46b3-68df-0310-9c12-caf00c1e9a41