I want to import contacts into Amplified, but will they be added twice if they eventually buy and create an account in Shopify?
No, they won't end up being added to Amplified twice.
If you import contacts into Amplified, their email address and phone number will be stored as part of their contact information. Once the contact has made a purchase and created an account with Shopify, that info will not be stored in Amplified. This is because it's already been saved as part of the previous import process. It won't duplicate any data.