Enable duplicate checking when storing uploaded entries in the merged table

[scog1234]

Right now, we check for duplicate entries, but don't actually do anything if we find them. What we should do is warn the user that some of the entries in their uploaded data are duplicates (in the sense of having the same Username & IDs) as entries that already exist in the table. They should then be asked whether they want to replace the old entries (in which case, we should call the routine update_duplicates in ingest_datasets_better.py), or keep the old entries and ignore the new entries (in which case we call the ignore_duplicates routine instead). I tried to get this to work during the workshop, but my HTML skills are not up to the task, so hopefully someone else can take over with this.