This seems like a simple problem but I can't seem to figure it out. I'd like to remove duplicates from a dataframe (df) if two columns have the same values, even if those values are in the **reverse order**. What I mean is, say you have the following data frame:

```
a <- c(rep("A", 3), rep("B", 3), rep("C",2))
b <- c('A','B','B','C','A','A','B','B')
df <-data.frame(a,b)

  a b
1 A A
2 A B
3 A B
4 B C
5 B A
6 B A
7 C B
8 C B

```

If I now remove duplicates, I get the following data frame: 

```
df[duplicated(df),]

  a b
3 A B
6 B A
8 C B

```

However, I would also like to remove the row 6 in this data frame, since "A", "B" is the same as "B", "A". How can I do this automatically?

Ideally I could specify which two columns to compare since the data frames could have varying columns and can be quite large.

Thanks!