#' dup.action(s0)
#'
#' Actions to apply to duplicate records.
#'
#' @param object data;
#' @param ... addditional arguments passed to
#'
#' \code{dup.action} is a family of functions similar to \code{na.action} that
#' provide behaviors for data objects when duplicate records are encountered.
#' Unlike \code{na.action}, \code{dup.action} generally refers to a record
#' containing multiple columns, rather than a single vector.
#'
#' \strong{dup.pass} does nothing and passes \code{object} untouched
#'
#' \strong{dup.first} removes all duplicates \strong{but the first}.
#'
#' \strong{dup.last} removes all duplicates \strong{but the last}.
#'
#' \strong{dup.omit} removes \strong{all} records that have duplicates, i.e. only unique
#' records remain.
#'
#' The \code{dup.action} family of methods does not preserve attributes. To
#' preserve attributes during subset, see \code{\link{dedup}}.
#'
#' @return
#' an object with the same class as \code{object} with the associated
#' dup.action applied.
#'
#' Since a common scheme is to have an attribute \code{dup.action} associated
#' with data, these are copied into the resulting data if they exist
#'
#' @seealso
#' \code{\link[base]{duplicated}}, \cr
#' \code{\link{dup.pivot}}
#'
#' @examples
#' x <- data.frame( a=letters[ sort(rep(1:4,2)) ], b=1 )
#'
#' dup.pass(x)
#' dup.first(x)
#' dup.last(x)
#'
#' \dontrun{
#' setDT(x)
#' x[ , b := 1:2 ]
#' setkey(x,a)
#' dup.action(x)
#' dup.first(x)
#' dup.last(x)
#' }
#'
#' @rdname dup.action
#' @name dup.action
#' @import data.table
NULL
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