duplicates() function

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