Only leave duplicated rows in a dataframe, with R

I have a dataframe that looks like this:

And I want to leave only those rows that are duplicated for col1 & col2, keeping BOTH rows:

I’ve been trying to do this by using unique() or distinct() or anti_join() but can’t figure it out.

>Solution :

Base R:

df[df$col1 %in% df$col1[duplicated(df$col1)],]

  col1 col2 col3
1  tn1    a    b
2  tn1    a    c