Also posted on the Z-perl comment section.

Getting these errors on login and /reloadui and it seems to concern ALL THE THINGS, when used with Z-perl unit frames. When I have both ALL THE THINGS and Z-Perl enabled at the same time the error happens.

If I turn one or the other off, it stops.

132x Deferred XML Node object named Lib_UIDropDownListTemplate already exists
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

Locals:
nil

132x Deferred XML Node object named Lib_UIDropDownMenuTemplate already exists
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

Locals:
nil

175x Deferred XML Node object named Lib_UIDropDownMenuButtonTemplate already exists
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

Locals:
nil

I get the same thing and don't use Z-Perl. Haven't debugged further yet.

Broker Keystone and Titan Panel have a similar problem with conflicting with each other. Turn one or the other off and it stops. Have nothing but the two enabled and the problem shows.

That is the two only sets of conflicting addons I have observed so far.

What other addons are you using and are they fully up-to-date including ATT 1.6.4a?

The reason I ask is none of the contribs or members of our discord has received this error. Also, your bug report looks like it's Bug Grabber giving off the error which is odd. Usually when the error is reported there is more to it with the name of the addons causing them.

I could turn on ATT again today without getting the error, so there must have been a fix in the latest versions of one or both of them after I reported the error here and on z-perl.

Someone posted something about it here as a reply to why it happens:
https://www.curseforge.com/wow/addons/zperl?comment=2839
"I did some digging and it turns out that Blizzard is now calling out an error if two different addons have the same lib packs in them. So any addon that shares a lib pack with another and includes it in their package will cause errors like this." - Branalia In reply to SiegaPlays:

So you can consider it solved.

Well we did some experimentation last night and TSM seemed to proc this bug.

It turns out that some addons such as TSM, etc don't like to have a /reloadui done to them and was causing the error. I'd suspect the same occurred for other addons.

Another update seems to be Blizzard has made it so an error occurs if two addons are using the same library. Rather than keeping the XML library ourselves causing issues with other addons. We will probably rework the settings menu (where it's used) and rebuild the drop down ourselves so there is no error.

Originally we used the XML library because when building a drop down menu with Blizzard's UI after 4 items you would receive a taint error which the library fixed.