Sequence Numbers with Dplyr

Question

Given the dataset below:

tmp = data.frame(id = c(1,1,1, 2, 2, 3),
             cont_date = c("2015-01-01", "2016-01-01", "2017-01-01", "2017-01-01", "2015-01-01", "2016-01-01"))
library(lubridate)
tmp$cont_date = ymd(tmp$cont_date)

I'm probably missing something obvious, but I was expecting the following code

tbl_df(tmp) %>% group_by(id) %>% arrange(cont_date) %>% mutate(seqnum = 1:length(id))

to create the sequence number of each block of records that are grouped by identifier. Instead, I get:

      id  cont_date seqnum
1  1 2015-01-01      1
2  2 2015-01-01      2
3  1 2016-01-01      3
4  3 2016-01-01      4
5  1 2017-01-01      5
6  2 2017-01-01      6

I do not want the sequence number of all entries, but for each block. It seems that part is being group_byignored.

Any help would be greatly appreciated.