Select similar and unique values in a data frame
df = data.frame(email_one=c("one@gkn.com","two@wern.com","three@fu.cin",
        "four@huo.com","five@hoi.com"), email_two=c("ten@hoinse.com",
        "four@huo.com","two@wern.com","five@hoi.com","six@ihoio.com"))

# Elements present in both columns
intersect(df[[1]], df[[2]])
[1] "two@wern.com" "four@huo.com" "five@hoi.com"

# Elements of column 1 that are not in column 2
setdiff(df[[1]], df[[2]])
[1] "one@gkn.com"  "three@fu.cin"

# Elements of column _2_ that are not in column _1_
setdiff(df[[2]], df[[1]])
[1] "ten@hoinse.com" "six@ihoio.com"