I see this all the time and I always report and I often also inform the person being impersonated.

Medium's security is pathetic. There is no reason that these accounts should ever be able to be set up. If the screen name already exists, Medium could require further verification before allowing the account to use it. If an account is using an already existing icon, it's also easy for that to be identified in real time and again, require further validation of a unique identity before allowing that icon to be used to set up an account or to be edited in after account creation.