Remove all unique rows

I am trying to figure out how to remove all unique rows, from a data frame, but if it has a duplicate, I want that to stay in. For Example - I want all columns from this with col1 the same: ``` df<-data.frame(col1=c(rep("a",3),"b","c",rep("d",3)),col2=c("A","B","C",rep("A",3),"B","C"),col3=c(3,3,1,4,4,3,2,1)) df col1 col2 col3 1 a A 3 2 a B 3 3 a C 1 4 b A 4 5 c A 4 6 d A 3 7 d B 2 8 d C 1 subset(df,duplicated(col1)) col1 col2 col3 2 a B 3 3 a C 1 7 d B 2 8 d C 1 ``` But I want to have rows 1,2,3,6,7,8 since they all have the same col 1. How do I get 1 and 6 to be included? Or, conversely, how do I remove rows that do not have a duplicate?