ShojiOrder/OrderGroup and
removes any duplicate entities. As with the default of
duplicated, the first appearance of an entity is kept,
and subsequent occurences are marked as duplicated and removed. (Unlike
duplicated, however, there is no option to reverse that order.)
"First" occurence of an entity is determined by the function's recursion:
within each group, nested groups are processed first, in order, and
recursively their nested groups are processed. See the test suite, in
test-variable-order.R, for an example that illustrates which entities are
dropped as duplicate.
dedupeOrder(x)x with duplicate entities removed.
duplicates, which when set to FALSE also calls
this function.