I've been copying a task (a customized panel - an extra tab in Sales Order - Invoice Data Entry) to multiple users, however, when copying the task to one particular user, I see an error that says: 

Unable to create the following new library, possibly because it already exists

Does anyone know about this and if it's possible to combine a previously customized panel with the new one? The new one just has a tab, which I selected in Customizer Selection (not the version ending in W, but the regular version of the custom tab itself, not the header or main). Is it possible to combine tabs somehow? I'm not sure what customizations this user has, but I don't want to clear them. 

support.na.sage.com/.../viewContent.do

This article says - Note: After successfully copying over the customizations, if any additional panel customizations need to be added back to the specific User/Group and Company, then manually add them.

I'm wondering about "manually add them" and if this means I would need to edit the panel again to add the other customizations this user has, or simply add the customizations back using the user's customized panel in Customizer Selection...

I'm wondering why this is the way it is, when the customization I'm adding is an entirely new tab that doesn't exist in the user's SO Invoice Data Entry task...

Also, do I need to copy both the W (wide) version of the custom task and the regular version? Or just one or the other? Is it typically best to just copy DMAIN or DMAINW instead of a more specific aspect of the task, like the new tab I created?