Werk #12634: check plugin register: raise error on duplicate plugin

This change is only relevant for customers who develop and maintain their own checks. In the event that the same check plugin exists in both check API paths (checks and cmk/base/plugins/agent_based), the legacy check plugin overrides the new plugin without generating an error or a warning. With this change, the API raises an error in the event that the check plugin has been duplicated.

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