Use SSH with multiple accounts

Here’s the situation: The easiest and most secure way to interact with Git repos hosted on GitLab is over the SSH protocol. While most people only use one account, a freelancer or consultant might need to work with repos from multiple accounts. If that freelancer attempts to upload their public SSH key to multiple accounts, they’ll get the following error message from GitLab: Fingerprint has already been taken. Assuming they don’t want to use password-based authentication over HTTPS as a workaround, how can our friendly freelancer get SSH working?