Вот одна идея.
library(tidyverse)
dat %>%
mutate(DATE = as.Date(DATE)) %>%
group_by(ID, Serial) %>%
summarize(VALUE = last(VALUE), DATE = min(DATE)) %>%
ungroup() %>%
arrange(ID, DATE)
# # A tibble: 5 x 4
# ID Serial VALUE DATE
# <dbl> <dbl> <dbl> <date>
# 1 1 11 -2 2019-10-01
# 2 2 22 -10 2019-09-01
# 3 2 12 9 2019-09-03
# 4 3 12 -8 2019-08-01
# 5 3 13 -7 2019-08-04
ДАННЫЕ
# Create an example
dat <- tribble(
~ID, ~Serial, ~VALUE, ~DATE,
1, 11, -1, "2019-10-01",
1, 11, -2, "2019-10-02",
2, 22, -9, "2019-09-01",
2, 22, -10, "2019-09-02",
2, 12, 9, "2019-09-03",
3, 12, -10, "2019-08-01",
3, 12, -8, "2019-08-03",
3, 13, -7, "2019-08-04"
)