structure(list(Case.Number = c("127967", "127967", "127967",
"127967", "141330", "141330", "141330", "141330", "141240", "141240",
"141240"), Word = c("account", "want", "membership", "sort",
"unhappi", "vr", "info", "miss", "csrf", "unhappi", "dissatisfi"
)), .Names = c("Case.Number", "Word"), class = c("data.table",
"data.frame"), row.names = c(NA, -11L), .internal.selfref = <pointer:
0x0000000000120788>)

127967 account want
127967 account membership
127967 account sort
127967 want    membership
127967 want    sort
141330 unhappi vr
141330 unhappi info...

excluding
141330 unhappi unhappi

source <- c("remove")
target <- c("remove")
ID <- c("remove")
df <- data.frame(ID = c("remove"), source = c("remove"), target = c("remove"))

for(i in unique(tbl$Case.Number)){
  for (r in grep(i, tbl$Case.Number)) {
    if(r < max(grep(i, tbl$Case.Number))){
      ID <- i
      source <- tbl$Word[r]
      target <- tbl$Word[r+1]
      rbind(df, cbind(ID, source,target))
    }

  }

}

View(df)