If an import has the same email address as an existing person, the existing person will be updated with any new information in the import. However, there is currently no way to avoid updating existing information.  To avoid importing duplicates, you can export the pipeline and use Excel or Sheets to compare the import with the export.  The import tool does not currently allow the import of notes, but this is an upgrade that is being worked on. The tool will only update existing fields if they are filled out in the CSV, and it will not reset the stage of imported contacts. You can confirm this by modifying a test contact and importing a CSV with just a header row and one data row. After the import, all existing data that was not in the import should still be on the prospect.