Может быть, это поможет
library(tidyverse)
df1 %>%
group_by(STUDID, MATSUBJ) %>%
summarise(SCORE = max(SCORE),
flagextra = as.integer(!sum(duplicated(MATSUBJ))))
# A tibble: 3 x 4
# Groups: STUDID [?]
# STUDID MATSUBJ SCORE flagextra
# <int> <chr> <dbl> <int>
#1 101 AFRIKAANSB 4 0
#2 102 ENGLISHB 5 0
#3 102 ISIZULUB 7 1
Или с base R
i1 <- !(duplicated(df1[1:2])|duplicated(df1[1:2], fromLast = TRUE))
transform(aggregate(SCORE ~ ., df1, max),
flagextra = as.integer(MATSUBJ %in% df1$MATSUBJ[i1]))
данными
df1 <- structure(list(STUDID = c(101L, 101L, 102L, 102L, 102L), MATSUBJ
= c("AFRIKAANSB",
"AFRIKAANSB", "ENGLISHB", "ISIZULUB", "ENGLISHB"), SCORE = c(1L,
4L, 2L, 7L, 5L)), class = "data.frame", row.names = c(NA, -5L
))