keeps only duplicates.
In DataCombine: Tools for Easily Combining and Cleaning Data Sets

dMerge merges 2 data frames and reports/drops/keeps only duplicates.

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dMerge(data1, data2, by, Var, dropDups = TRUE, dupsOut = FALSE,
  fromLast = FALSE, all = FALSE, all.x = all, all.y = all,
  sort = TRUE, suffixes = c(".x", ".y"), incomparables = NULL)

`data1`	a data frame. The first data frame to merge.
`data2`	a data frame. The second data frame to merge.
`by`	specifications of the columns used for merging.
`Var`	depricated.
`dropDups`	logical. Whether or not to drop duplicated rows based on `Var`. If `dropDups = FALSE` then it gives a count of the duplicated rows.
`dupsOut`	logical. If `TRUE` then a data frame only containing duplicated values is returned and `dropDups` is ignored.
`fromLast`	logical indicating if duplication should be considered from the reverse side. Only relevant if `dropDups = TRUE`.
`all`	logical; all = L is shorthand for `all.x = L` and `all.y = L`, where `L` is either `TRUE` or `FALSE`.
`all.x`	logical; if TRUE, then extra rows will be added to the output, one for each row in x that has no matching row in y. These rows will have `NA`s in those columns that are usually filled with values from y. The default is #' `FALSE`, so that only rows with data from both x and y are included in the output.
`all.y`	logical; analogous to `all.x`.
`sort`	logical. Should the result be sorted on the by columns?
`suffixes`	a character vector of length 2 specifying the suffixes to be used for making unique the names of columns in the result which not used for merging (appearing in by etc).
`incomparables`	values which cannot be matched. See `match`.