Select groups based on number of unique / distinct values

Here’s a solution using dplyr:

library(dplyr)

sample <- data.frame(
  ID = 1:9,  
  Group= c('AA', 'AA', 'AA', 'BB', 'BB', 'CC', 'CC', 'BB', 'CC'),  
  Value = c(1, 1, 1, 2, 2, 2, 3, 2, 3)
)

sample %>%
  group_by(Group) %>%
  filter(n_distinct(Value) == 1)

We group the data by Group, and then only select groups where the number of distinct values of Value is 1.