[Rd] duplicates() function

[Duncan Murdoch]

I need a function which is similar to duplicated(), but instead of 
returning TRUE/FALSE, returns indices of which element was duplicated.  
That is,

 > x <- c(9,7,9,3,7)
 > duplicated(x)
[1] FALSE FALSE  TRUE FALSE TRUE

 > duplicates(x)
[1] NA NA  1 NA  2

(so that I know that element 3 is a duplicate of element 1, and element 
5 is a duplicate of element 2, whereas the others were not duplicated 
according to our definition.)

Is there a simple way to write this function?  I have  an ugly 
implementation in R that loops over all the values; it would make more 
sense to redo it in C, if there isn't a simple implementation I missed.

Duncan Murdoch