Importing WorkItems wipes all previous data

When I import a new set of work items it wipes out the previous set of workitems however I could be importing a new batch.

I'd like the import process to dedupe against data already in there.

Start at the WorkItems import process and inspect how incoming items replace the existing set. Define how duplicates are identified from the current data model, then verify that importing a new batch preserves previous items and avoids duplicate entries.

Written by the indexing model from the issue text.