Unique on a dataframe with only selected columns

Question

I have a dataframe with >100 columns, and I would to find the unique rows, by comparing only two of the columns. I\'m hoping this is an easy one, but I can\'t get it working

Answer 1

Minor update in @Joran's code.
Using the code below, you can avoid the ambiguity and only get the unique of two columns:

dat <- data.frame(id=c(1,1,3), id2=c(1,1,4) ,somevalue=c("x","y","z"))    
dat[row.names(unique(dat[,c("id", "id2")])), c("id", "id2")]