I have an example data set like this:

```r

df1 <- data.frame(c1=c('a','b','c','d','e','f','g', 'h'),
         c2=c('l','m','a','g','e','q','a','d'))

```

and I just want a data frame that removed the duplicates between c1 and c2. I already know how to grab the unique elements from c1 and c2, but what do I do after that, to end up with something like the following:

```r
data.frame(c1=c(b,c,f,h),c2=c(l,m,q,NA))

```