Allow duplicate import of data

Some people may have an import fail part way through, or want to reimport to update as new features are added. We need to support this.

We need to be saving the IDs from SimplyPlural to be able to match up to prior imports, to not duplicate data the best we can.

For imports that have already happened, we need to do our best to deduplicate the data so we don’t end up with double imported data.