Duplicate events broadcasted to same API server

Describe the bug When an API server is configured in both a events_observer block in the stacks-node's config TOML, and the STACKS_EVENT_OBSERVER environment variable, stacks-node will broadcast transactions to the target API server twice, causing it to fail over time.

Steps To Reproduce

1. Start a stacks-blockchain-api server
2. Export the STACKS_EVENT_OBSERVER env variable pointing to the API server.
3. Run stacks-node with a config TOML which has a events_observer block pointing to the API server

Expected behavior The stacks-node should detect if there are duplicate event server entries between the events_observer block in the TOML and the STACKS_EVENT_OBSERVER env variable. If duplicate entries are found, print a log message saying so and exit.

Environment (please complete the following information): stacks-node version 2.0.11.2.0

More info can be found here.