I'm tempted to say "it's easy" but: -
It's years since I did any proper Web development
If it was easy, Invision (no matter what sarky comments from members) would have fixed it; they are a commercial company who rely on reputation...
It shouldn't (repeat Shouldn't) be difficult, when merging posts from the same poster, be hard to check "is the text identical to last time?" (bearing in mind "last time" is the same poster within the last 60 minutes; and the first post is already in the DB otherwise you would not be merging...). If so, quietly drop...
If it is difficult, bury a hidden token in the post (needn't be large, again, it's the same poster in a short time period) which flags "this is the same post, repeated..."
But, I suspect, if it was easy, Invision would have already sorted it...