When attempting to add a deployment key to a GitHub repository, such as in the case of needing access for a submodule, or fetching remote dependencies within your build pipeline, you may run into the following error:
Key is already in use
This is because your DeployHQ project's public key has already been added to GitHub, most likely as the deployment key for the main repository you're trying to deploy.
Github does have a restriction that only allows a deployment key to be used on a single repository, but they offer a solution whereby you can create a machine (non-human) user and assign the deployment key to that user, which will then be given access to multiple Github repositories.
More information on this can be found in Github's documentation.