The main function of the Force Resync button is to populate new merge mapper data. For example, if you add a new merge tag in Mailchimp and want to map it from a Shopify field. Another use for it would be to restart the sync process if there was an issue.
If you click the "Force Resync" button, it will not duplicate your data. If any new data is found in Shopify, it will merge with your Mailchimp list.