Today I've come across at least 4 sets of duplicate posts. I've never seen this happen here before. There is one set here:
The thread "Moringa" in Classifieds and Group Buys forum

I'm still seeing duplicated posts. Here was one from today:
The thread "Grumpy Thread 2016" in Sandbox forum
It was Linda's post.

I'm pretty sure this has nothing to do with the system. It's the way the member posts. I don't know whether it's a case of double-clicking or hiccups or just a sneaking suspicion that the post didn't go through the first time.

Linda's posts are almost always doubled. It's so common that I sometimes feel concerned when I see only a single post.

Ok, you'd know better than I do. I remember it used to happen on Ebay forums when people would post and then keep refreshing to see if anyone answered.

I don't think that's the case here. Both of Linda's posts were sent in the same minute, so it's unlikely that she would have expected an answer that soon.

I hope @Dave can tell us what causes this, so more don't do it.

Much easier to ask Linda what she does to produce all of these duplicate posts. @LindaTX8

Ha, sorry, Linda! I have seen a number of other people with them, too. She's definitely not alone.

In my experience, a lot of people tend to double-click web links, buttons, etc. when they only need to click once (they know they have to double-click everywhere else in Windows to open things, so the single-click web stuff is a bit of an exception to the rule and something that easily catches some people out).

For many sites, this means that the form is submitted twice, leading to double posts. I suspect that's all this is. Perhaps @dave can modify the server-side code to check for duplicate submissions?

I used to see double posts from someone posting from an older IPad.

It also seems to happen to folks that have devices synced. If their iPad is logged into NGA and they post form their phone, sometimes it seems to send the post from each. Weird...

Thanks to all of you for your ideas about this. I can see that maybe it is a double clicking scenario. The thing that puzzles me the most is that I never saw this happening anywhere on the site, and the last few days it's happened on numerous posts. Something has changed.

Maybe I just have a double-clicking brain! Nothing to do about that, although I'd love to have the brain I used to have in the 70's...would need all I've learned since then transferred over to the new, better brain. Maybe the computer I'm using, which I'll get rid of as soon as I can get someone to set up my new one. Or my mouse, which is pretty old...not as old as I am, however. Will have a new mouse when the new computer is set up.