How to add a key to a repository on Bitbucket if it says "Someone is already using this key"?

Question

Bitbucket

When trying to add an SSH key to Deployment Keys, it says "Someone is already using this key". How to find out who this someone is? And why is he using my key?
And the second question, it says that:

Deployment
keys Use deployment keys to get read access to this repository. Learn more about using SSH keys.

...type key for read access. This key will not give me the opportunity to push?