duplicated.data.frame {was "[R] which rows are duplicates?"}

    WK> what the documentation *fails* to tell you is that the parameter
    WK> 'incomparables' is defunct

No, not "defunct", but the contrary of it,
"not yet implemented" !
  

    WK> # data as above, or any data frame
    WK> duplicated(data, incomparables=NA)
    WK> # Error in if (!is.logical(incomparables) || incomparables)
    WK> .NotYetUsed("incomparables != FALSE") :
    WK> #   missing value where TRUE/FALSE needed

    WK> the error message here is *confusing*.  
yes!
  

    WK> the error is raised because the
    WK> author of the code made a mistake and apparently haven't carefully
((plural or singular ??))
  

    WK> examined and tested his product;  the code goes:
((aah, ... "singular" ...))
  

    WK> duplicated.data.frame
    WK> # function (x, incomparables = FALSE, fromLast = FALSE, ...)
    WK> # {
    WK> #    if (!is.logical(incomparables) || incomparables)
    WK> #        .NotYetUsed("incomparables != FALSE")
    WK> #    duplicated(do.call("paste", c(x, sep = "\r")), fromLast = fromLast)
    WK> # }
    WK> # <environment: namespace:base>

    WK> clearly, the intention here is to raise an error with a (still hardly
    WK> clear) message as in:

    WK> .NotYetUsed("incomparables != FALSE")
    WK> # Error: argument 'incomparables != FALSE' is not used (yet)

    WK> but instead, if(NA) is evaluated (because '!is.logical(NA) || NA'
    WK> evaluates, *obviously*, to NA) and hence the uninformative error message.

    WK> take home point:  rtfm, *but* don't believe it.

and then be helpful to the R community and send a bug report
*with* a patch if {as in this case} you are able to...

Well, that' no longer needed here,
I'll fix that easily myself.