feat(query): config option to allow for showing a warning for repeat listeners

[prescottprue]

Do you want to request a feature or report a bug?
Feature

(If this is a usage question, please do not post it here—post it on gitter. If this is not a “feature” or a “bug”, or the phrase “How do I...?” applies, then it's probably a usage question.)

What is the current behavior?
Sometimes there are repeat listeners which can cause unexpected behavior - if a config option allowed for a warning to be shown if there are two listeners active for the same path, this could be solved.

This would have made debugging much easier for a bug we (Side Inc. Engineering experienced. Thanks to @lautapercuspain and @urbantumbleweed for uncovering the bug.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via codesandbox or similar.

What is the expected behavior?
If warnForRepeatedListeners: true is set in config, a warning appears if two listeners with the same settings are attached

Which versions of dependencies, and which browser and OS are affected by this issue? Did this work in previous versions or setups?