[R] "conditional duplicates"?

[Christian Schulz]

Hi,

i would like check a repeated measurement dataset, whether
cases (which (id) could be more than one time included and not all same 
times included.)
have more than one times the same SMONTH!?

#This pseudo code didn't work with a for loop, because the [i+1] isn't 
known.
How i could refer to [i+1] ?

if(ID[i] == ID[i+1] & SMONTH[i] !=[i+1])  {res[i] <- 0 }
if(ID[i] == ID[i+1] & SMONTH[i] == [i+1]) {res[i] <- 1 }
 
many thanks,
christian