These messages can be displayed whenever target list emits a target a bit too early and the resource watcher ends up processing it twice.

Until we actually fix this flaw in the TargetList design, we should only log warnings at https://searchfox.org/mozilla-central/rev/f47a4b67643b3048ef9a2e2ac0c34edf6d1ebff3/devtools/shared/resources/resource-watcher.js#753

Definitely a good first bug for anyone who wants to setup the development environment.

To be really explicit about the expected change here:

      console.error(
        `Already started legacy listener for ${resourceType} on ${targetFront.actorID}`
      );

should be changed from error() to warn():

      console.warn(
        `Already started legacy listener for ${resourceType} on ${targetFront.actorID}`
      );

https://searchfox.org/mozilla-central/rev/f47a4b67643b3048ef9a2e2ac0c34edf6d1ebff3/devtools/shared/resources/resource-watcher.js#752

Hello Julian, I would like to work on the issue.

Hi Rahul! Thanks for proposing to work on this bug, assigning it to you right now.

If this is your first contribution to firefox and you need to setup your development environment, take a look https://firefox-source-docs.mozilla.org/contributing/index.html and don't hesitate to ask around on https://chat.mozilla.org if you are stuck

For the change itself there's not much more to it than what I mentioned in comment 1, but again don't hesitate to ask if you have any question.

You can also use the "Request information from" field below this comment box. It's the best way to notify people when you have a question.

Sure Julian. Will start to work on it.

https://hg.mozilla.org/mozilla-central/rev/904e4e21363c