Today I've noticed something strange with. If try to post the same thing (same post) it didn't check as usual for a duplicate post and it's added to the original message. Can someone confirm this ?

Nope, trying that I get the following (as expected) ;

Turn them on one by one until it breaks ?

Yeah, but how do I uninstall the thing it did? Because when I disable it, it doesn't come undone.

Also, is there any good way to narrow the search? Because I have a few dozen products installed, including arcade, url rewrite, Itrader... It would take a long time to disable and check all of them, and members would notice.

I am going to uninstall this since it is not working and I have noticed my server load has increased.

by how much has it increased?

Disabling and uninstalling this are the same thing as its purely plugin based. The most likely reason it breaks is either you have a custom style (with custom editor_clientscript) or some other plugin is editing the editor clientscript. Impossible to say. This appends code to the editor_clientscript template (code).

It works fine, and has no effect on server load, so I would suggest you look at your server more closely.

Now, see, that just can't be right... It was the vBulletin Link Canonicals! ROFLMAO!

Is that a mod ?

In think that could be a problem with some of my users connection (bouncing or something) because even when I was not using your mod, I've noticed some double (exactly the same) posts for some users, even with Vb flood checking like you mentioned. But always when I tried to reproduce the only thing I can get is vB message of double post.