search for: subjectnumb

Hi, I'm trying to find out if there is a command that tells me which cells in my data frame have the same value. I am looking at the results from an online survey, organized into a data frame with the following columns: SubjectNumber, WorkerID, Age. I know the one person must have taken the survey more than once, because there is one less level in the WorkerID column than there are rows in the data frame. Is there something that will tell me which SubjectNumber cells correspond to more than one WorkerID cell? Thanks, Stev...