Extract rows that have common values in R dataframe

Question

This seems very simple problem but could not figure out or find a solution in OS (this is not duplicate). In an example dff below, I want to extract events that are common in both 5 and 6 (mn value), i.e., In this example, how to extract dff to get only Event2 in a new dataframe.
dff<-structure(list(ev = c("Event1", "Event1", "Event1", "Event1", 
    "Event1", "Event1", "Event1", "Event1", "Event1", "Event2", "Event2", 
    "Event2", "Event2", "Event2", "Event2", "Event2", "Event2", "Event2", 
    "Event2", "Event2", "Event3", "Event3", "Event3", "Event3"), 
    mn = c(5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 
    6, 6, 6, 6, 6, 6, 6)), row.names = c(NA, -24L), class = c("tbl_df", 
                           "tbl", "data.frame"))
dff

ThomasIsCoding · Answer

A base R option using aggregate + unique
> aggregate(. ~ ev, unique(dff), function(x) length(x) > 1)
      ev    mn
1 Event1 FALSE
2 Event2  TRUE
3 Event3 FALSE

akrun · Answer

Or an option with data.table
library(data.table)
unique(setDT(dff)[dff[, .I[uniqueN(mn) > 1], ev]$V1])

d.b · Answer

dff %>%
    group_by(ev) %>%
    filter(length(unique(mn)) > 1) %>%
    unique()

OR maybe
dff %>%
    group_by(ev) %>%
    summarise(chk = all(c(5, 6) %in% mn))

Extract rows that have common values in R dataframe

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