How does Dupe Bugs work?

Well hello,
I am already reading since a year or so here, but now I have a question.

I am playing a Online Game called Florensia and I knew there are some Dupe Bugs.
I knew where they are working and the rough outline how it works but I am not able to do it.

And since I am technical interested, I wanted to ask how Dupe Bugs generally works.

I thought, that its an Missing Transaction Statement in the Database, so that it doesn´t know after an error what to do with the Transaction and maybe keep an "copy" of the money/item.

So is there anyone who could explain me how something like that works (and do it in practical) and how its possible to get an Server-Error?

Dupe bugs generally work with packets. You can either blocked the packet where you "lose" the item, or you manage to create 2 "drop" item packets, or 2 pickup packets etc. You need to learn how to use a PE of your choice. There's many, a community favorite seems to be WPE Pro.