Your code is fine, and produces the expected results (at least on my setup)
library(tidyverse)
df <- tibble(ID=c(1,1,1,4,5,6,6,5,1),
Cluster=c(1,1,4,8,2,4,7,8,7))
df%>%
group_by(ID) %>%
summarise(Cluster=paste0(sort(unique(Cluster)),
collapse=", "))
# A tibble: 4 x 2
ID Cluster
<dbl> <chr>
1 1 1, 4, 7
2 4 8
3 5 2, 8
4 6 4, 7