When importing contacts into CiviCRM, you have the option of adding those contacts to a group and/or assigning them a tag. This will only apply to NEW contacts being imported, not to duplicate contacts that may be found and skipped or updated during import. A fool-proof method to identifying imported contacts (e.g. in order to be able to easily search and/or delete them if a mistake has been made) is to include a custom field for this purpose. I like to call it “Import Identifier”.
No matter how many controls we put into place to prevent duplicate contact creation, inevitably we always end up with a few, especially when website traffic volume has been high. As such, best practices for managing contacts include a periodic review and merging of duplicate contacts (especially after a big event or fundraising campaign). CiviCRM is equipped with several features that help prevent duplicate contact creation.
Does your data contain special characters such as accented letters or names with apostrophes? Have you exported that data in CSV format and found that those special characters have been made even more special (funky, actually)?
If so, there is a setting called "Language Encoding" located in CiviCRM under Administer > Localization > Languages, Currency, Locations that can be configured as "UTF-8" such that any special characters being exported to CSV will be preserved when you open the file.