我已经能够用下面的代码在Excel中执行“ctrl+d”：

library(RDCOMClient)

xlApp <- COMCreate("Excel.Application")
xlApp[["DisplayAlerts"]] <- FALSE
xlApp[["Visible"]] <- TRUE

path_To_Excel_File <- "D:/excel_File.xlsx"
xlWbk <- xlApp$Workbooks()$Open(path_To_Excel_File)
sheet <- xlWbk$Sheets(1)
rng <- sheet$Range("B2")
rng$select()

# see https://www.contextures.com/excelvbasendkeys.html for different keys
# see http://cpap.com.br/orlando/SendKeysMore.asp for different keys
# see https://software-solutions-online.com/vba-application-sendkeys/ for different keys
# ^ means ctrl
xlApp$SendKeys("^d", FALSE)