Upgrade assistant shows duplicate plugin names for fetch_error deprecation level

[jportner]

Kibana version: 7.16 (unreleased)

Describe the bug:

If a plugin registers multiple fetch_error level feature deprecations, the callout at the top of the Upgrade Assistantw ill show the plugin name multiple times.

Steps to reproduce:

1. Start Kibana using this branch: Register kibana_dashboard_only_user and kibana_user roles deprecations in UA. #110960
2. Add a new role that grants only the manage ES cluster privilege
3. Add a user that has the new role and the kibana_admin role
4. Log in with the new user and access the Upgrade Assistant
5. The security plugin will try to fetch users twice, and it will try to fetch role mappings twice. All four attempts will fail, and the callout at the top will mention the security plugin four times.

Expected behavior: The callout should deduplicate plugin names.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/kibana-stack-management (Team:Stack Management)

[cjcenizal]

This sounds like a duplicate of #111854, which was fixed by #111888 a couple weeks ago (after the referenced PR was opened). @jportner Could you try bringing that PR up to date with 7.x and verifying this bug still exists?

[jportner]

Oh dang, yes that looks to be the case, sorry for the duplicate!

[cjcenizal]

Thanks @jportner!