Skyp will check each upload and if a contact exists it will simply include that contact in the current campaign without uploading an exact duplicate.
IF there is Additional Context for the contact already, it will not be overwritten. The new context will be ignored.
To overwrite additional context:
Delete the existing additional context by editing the contact in Skyp.
Either manually overwrite the additional context field, OR
Upload a list including that contact's email and new additional context. If additional context is blank, it will import the new context and associate it with that contact