Replace value in a row based on duplicat ID and delete subsequent rows in R -

i have database following structure:

id <- c(1,2,3,4,4,5,6,7,7,8) var1 <- c(1,2,1,2,4,1,2,3,5,4) var2 <- c(6,8,7,4,7,8,9,5,7,5) df <- data.frame(cbind(id,var1,var2)) 

the dataframe looks now:

   id var1 var2 1   1    1    6 2   2    2    8 3   3    1    7 4   4    2    4 5   4    4    7 6   5    1    8 7   6    2    9 8   7    3    5 9   7    5    7 10  8    4    5 

i want replace var2 value first duplicated id var2 value second duplicated id (see lines 4:5 , 8:9) , delete entire row second duplicate id. final df this:

  id var1 var2 1  1    1    6 2  2    2    8 3  3    1    7 4  4    2    7 5  5    1    8 6  6    2    9 7  7    3    7 8  8    4    5 

this should work (note op not specific more 2 duplicates, take first var1 , last var2):

library(data.table) dt = data.table(df)  dt[, list(var1 = var1[1], var2 = var2[.n]), = id] #   id var1 var2 #1:  1    1    6 #2:  2    2    8 #3:  3    1    7 #4:  4    2    7 #5:  5    1    8 #6:  6    2    9 #7:  7    3    7 #8:  8    4    5