df = structure(list(Group = c(1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 3, 3,
3), index = c(1, 2, 3, 4, 1, 2, 3, 4, 5, 6, 1, 2, 3)), row.names = c(NA,
-13L), class = c("tbl_df", "tbl", "data.frame"))

df = structure(list(Group = c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1), index = c(1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4,
4, 4, 4)), row.names = c(NA, -16L), class = c("tbl_df", "tbl",
"data.frame"))

df = structure(list(Group = c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1), index = c(1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, 1,
2, 3, 4)), row.names = c(NA, -16L), class = c("tbl_df", "tbl",
"data.frame"))

library(dplyr)

df %>%
  group_by(Group) %>%
  slice(rep(seq_len(n()), each = n()))

df %>%
  group_by(Group) %>%
  slice(rep(seq_len(n()), n()))