To fix this, remove the deploy key from the repository, and add the key to your user account instead.
If the key you are using is intended to be a deploy key, check out our guide on deploy keys for more details.
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Error: Permission to user/repo denied to user/other-repo
This error means the key you are pushing with is attached to another repository as a deploy key, and does not have access to the repository you are trying to push to.