I want to extract all pairs within groups of a data.table by two columns.

I have the following code:

dt = data.table(g1 = c(1, 1, 1, 2, 2, 2, 2),
                g2 = c(3, 3, 4, 5, 5, 6, 6),
                values = letters[1:7])

dt[, .(ind = combn(values, 2, paste0, collapse = "")), by = .(g1, g2)]

But this returns the error

Error in combn(values, 2, paste0, collapse = "") : n < m

The ideal result is some object that contains groups such as:

a,b
c
d,f
g,h

If c was not included because there is only one value, that is not a huge concern to me.

Is this possible to get using data.table?

Thanks in advance.