Here is exactly what a user on my website stated on how to recreate this problem:
To replicate such problems with spoilers. Create a thread of some sort that includes a list of things. I.E.
"The Art of Sandwich making" Only the finest chef's in the Krusty Krab can make the best sandwiches by performing these steps:
Apply my buns
apply cheese please
Then proceed to be evil about it.
stealz from the krusty cube
Then proceed to post the thread, go back and edit that thread and adjust by adding some more text or removing a word and resave your newly changed thread. You may notice your newly posted thread will contain a duplicate. "How to make a kirby perty" and the spoiler will not only duplicate itself but find a way to cut part of the first spoiler's content in a portion to itself.
The problem seems to be when you are in the editting portion. Possibly mistakes between the rich text editor and the BBCode editor conversion code.
Well i just tested it and it'll appear as simple as "Edit -> More Options -> Preview" so it seems to happen along those lines as well. Didn't even have to switch editors.
Unfortunately that doesn't really help. I'm trying to get the BB code in the state that triggers the bug. The bug will be related to the rich text editor parsing in some way (there are a few parts). By getting the BB code in the state before it happens (but when it would happen if edited), I will be able to reproduce it. I haven't been able to reproduce it with th example post.
Alternatively, if there's a post where it is happening, you may be able to use the edit history tool to get the original version of the post (before editing) with the raw BB code. That could help me reproduce it.