[R] Simple question on finding duplicates

[Jeff]

   I'm  trying  to find duplicate values in a column of a data frame. For
   example, dataframe (a) below has two 3's. I would like to mark each value of
   each row as either not being a duplicate of the one before (0), or as a
   duplicate (1) - for example, as in dataframe (b). In SPSS, I would simply
   compare each value to it's "lagged" value, but I can't figure out how to do
   this with R.
   Can someone point me in the right direction?
   Thanks
   a <- data.frame( col1 = c(1,2,3,3,4))
   b <- data.frame( col1 = c(1,2,3,3,4), duplicate = c(0,0,0,1,0))