Importing Tags and Groups.
If you have multiple tags and groups to import that identify overlapping subsets of your data you will not be able to import them at first pass.
First import all of your data ignoring the tags and groups so that you get a full and clean set of data in CiviCRM.
You can import these by identifying each unique subset e.g. all those to be tagged 'xxx'. Create a new import file with just sufficient columns to identify the data and prevent duplication 'FirstName', 'LastName' and 'Email' will suffice for CiviCRM's default duplication detection.
Import this group with 'On Duplicate Entries' set to 'Fill,' so that exisiting data is not over-written, and use the 'Add to Group' and/or 'Add Tag' options in Step 3 to enter these records.
Note: you can add a new Group from this screen but not a new Tag. If the Tag does not already exist open Administer CiviCRM in a new window, create the tag, then go back and refresh the import screen and you will find the tag on the list (this saves restarting the import).