Fix error non numeric argument to binary operator

Duplicate sections within a CI log are possible, and those currently lead to fatal errors …

… in `dplyr::transmute()`:
… Caused by error in `end - start`:
! non-numeric argument to binary operator
… Values from `timestamp` are not uniquely identified; output will contain list-cols.`

This MR fixes those problems by defining the cols data types and summing up the durations of duplicates.