How can I duplicate groups of rows in a data.table when each group needs duplication a different number of times?

This is a double join, the first with id == apple, the second with id == pear, measure == measure. (And then a rename and optional column reordering.)

dt1[dt2, on = .(id == apple), nomatch = NULL] |>
  dt1[i = , on = .(id == pear, measure == measure)] |>
  setnames(c("id", "i.id", "value", "i.value"),
           c("pear", "apple", "pear.val", "apple.val")) |>
  [, c("type", "i.type") := NULL ]
     pear measure pear.val  apple apple.val
                
1:      d       1        1      a         1
2:      d       2        8      a         5
3:      d       1        1      b         1
4:      d       2        8      b         9
5:      f       1        9      c         4
6:      f       2        5      c         2

The order of the columns is different but it's the same data.