prevent assignment of duplicate labels to agents

[jameslamb]

• adds new tests (if appropriate)
• add a changelog entry in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

What does this PR change?

This PR attempts to prevent the situation where an agent has duplicate labels.

Why is this PR important?

While testing #3000 , I noticed something surprising...it's possible for duplicates to exist in an agent's set of labels:

For example:

prefect agent start \
    -t ${PREFECT_RUNNER_TOKEN} \
    --label 'i-am-duplicated' \
    --label 'i-am-duplicated' \
    --label 'i-am-duplicated'

I'm not sure if this would cause any problems today, but I could see it causing these types of problems:

• breaking code that allows wildcards for matching flows to agents, but throws an error if more than one label is matched
• breaking code that queries for agents by label and cannot handle duplicates
• slightly hurting the experience in the Prefect Cloud UI if duplicate labels are enough to push other labels into the collapsed +n box like in the screenshot above

So, I'm not aware of anything that is not working well today because of this, but I think this change might be a minor way to prevent some weird cases in the future.

Thanks for your time and consideration.

[joshmeek]

I think this change won't affect anything but pinging @zdhughes if he knows what would happen anyway when using duplicate labels. e.g. is a flow and agent with labels A,A different than a flow and agent only with the label A 🤔

[zdhughes]

Just confirmed-- label matching for run retrieval is done by way of sets, so the list ["foo", "foo"] gets collapsed down to the set {"foo"} on the API side. It looks like the same is not true for label-based flow run concurrency, but I think that's likely an oversight on the Cloud side. 👍

[joshmeek]

@zdhughes Awesome thanks for clarifying! Since this is only agent related it's good to go 👍