Maybe prefixing with "Previous discussions" instead of "Posted here multiple times before" will not sound like complaining

It's really hard to find a phrase for this that people don't interpret as a complaint or criticism. It took me years before I hit on "Related", which seems just neutral enough to mostly avoid that misunderstanding (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...).

The phrasing was matter of fact. And there is nothing inherently wrong with reposting. Certain other websites have developed a culture opposed to it, but I have yet to see that here.

Agreed, I actually tried and go back to change the wording to exactly that, but unfortunately it was already past the editing grace period. But lesson learned for next time.