Extract duplicate records in dataframe [duplicate]

Question

I have a data frame that I want to select both rows that have duplicated values. In the example below, I want a new data frame or two separate data frames with the two records for 19 and 32 respectively.

        a <- c(8, 18, 19, 19, 20, 30, 32, 32, 58)
        b <- c(1950, 1965, 1971, 1981, 1999, 1969, 1994, 1985)
        df <- data.frame(a,b)    
        df
        a    b
        1  8 1950
        2 18 1965
        3 19 1971
        4 19 1981
        5 20 1999
        6 30 1969
        7 32 1994
        8 32 1999
        9 58 1985  

I have tried using df[duplicated(df$a), ] but this only extracts the second record that is duplicated, where I want both of them. The end goal is to subtract the years in the second column between the two records of 19 and 32.

Answer 1

We can use

df[duplicated(df$a)|duplicated(df$a, fromLast=TRUE),]
#  a    b
#3 19 1971
#4 19 1981
#7 32 1994
#8 32 1999