duplicates in multiple columns

It works if you use duplicated twice:

df[!(duplicated(df[c("c","d")]) | duplicated(df[c("c","d")], fromLast = TRUE)), ]

  a  b c    d
1 1  2 A 1001
4 4  8 C 1003
7 7 13 E 1005
8 8 14 E 1006