I ran into this on a community board last week where someone kept replying to their own thread with quick fixes instead of using the edit button. Here is what actually worked without making things awkward.
Start with a compliment or acknowledgement, then ask directly. Something like, "Hey, solid point about the daily promo rotation. Would you mind hitting edit on your first post so the thread stays clean for people checking on mobile?" Most folks just do not realize the edit window is open for a full 24 hours on many forums, or they think double posting bumps the thread higher.
If they keep doing it, send a private message instead of calling them out publicly. A short note like, "No worries, just a heads up that merging your updates into the original post helps the search function find your answer later" tends to land better than a public scolding. I once saw a mod handle this by editing the duplicate themselves and leaving a tiny note: "Merged your update into the OP, thanks for the follow-up." That set the norm without a lecture.
For threads about XL live arcade games or questions like where to play Pragmatic Play slots online, double posts bury the actual answers. Same with an online slots leaderboard discussion — if three replies are all from the same person, newcomers skip the thread entirely.
One concrete trick: reply to the duplicate with just the edit link or a screenshot of the pencil icon. No sarcasm. It takes ten seconds and usually fixes the habit permanently.