I'm not currently sure if this is something we care about, but it is somewhat contrary to how the externallinks table/LinkSearch works, which logs every individual instance of even the same link on a page.

If a page has a link (e.g. https://google.com) and the same link is added to the page again, an event isn't fired.

I think we should fire an event here.

Likewise adding one link three times to a page only registers the link being added once.

But, I'm not sure of the utility of emitting the same link three times.

We talked about this in one of our meetings, and decided that this feature is not critical for the MVP. In the long run when more editors get involved with the article, I suppose that duplicate links will be consolidated into a single ref tag and referenced in multiple places on the page.

When we get around to working on the task, we should decide whether firing events when the link already exists on the page is a good idea. Contrary to what I said earlier in the previous comment, the stream should not be used to compute the state of the links at a given point, but rather to output incoming/outgoing unique links.

Instrumentation for the mediawiki.page-links-change event lives in the EventBus extension, not WikimediaEvents.