Command `config add` for additional URLs does not check presence of URL in `board_manager` tree

[ubidefeo]

Describe the bug
When manually adding an additional url via arduino-cli config add ... the command does not check for existing matching entries in the list, hence generating multiple entries for the same URL

To Reproduce
Steps to reproduce the behavior:

1. arduino-cli config add board_manager.additional_urls https://my.url/package_mycustom_index.json
2. arduino-cli config dump
3. verify that our URL is at the end of the board_manager group of additional_urls
4. arduino-cli config add board_manager.additional_urls https://my.url/package_mycustom_index.json
5. arduino-cli config dump
6. verify that the URL is twice at the end of the board_manager group of additional_urls
7. arduino-cli config remove board_manager.additional_urls https://my.url/package_mycustom_index.json
8. remove will remove any istance of said URL

Expected behavior
The command should check for the presence of said URL and not add it if already there

Screenshots
n/a

Desktop (please complete the following information):

• OS: Mac OS
• Version: 11.5.2

Additional context
n/a

[per1234]

Hi @ubidefeo. Shouldn't this issue be in the arduino/arduino-cli repository?

[silvanocerza]

It certainly should. Moving it.

[ubidefeo]

@per1234 @silvanocerza
of course it was :D

[henrygab]

Almost a year. I also hit this.

Question: If a PR was presented to fix this, what is the preferred fix?

Option #1. Apply this only to board manager URLs
Option #2. Apply this to any configuration setting that is a list
Option #3. Create a new sub-command for config, such as ensure

Looks like the place to add the logic (for first two options, or replicate for option 3) would be here:

arduino-cli/cli/config/add.go

Line 57 in 9c784d8

v = append(v, args[1:]...)