"and the only common thing here is Lacerte. This is a Lacerte issue."

Yes, it indicates it is having a hard time tracking what you are doing.

"Have you experienced this specific error?"

Yes and no. There are pointers to where files are found. The reason you were asked about external backups, cloud storage, and if you were using a backup routine (outside of Lacerte, such as Carbonite, Mozy, Dropbox, a backup drive that has a built-in scheduler) is that these tools mirror, they don't backup, the files.

Everything I use/support that comes from Intuit has a backup function in the program and you don't want to use a mirror function when what you want is a backup function, unless you use the program to backup, and the mirror function to "save more than one copy of this somewhere else." You mentioned using Restore, so this helps identify you know to restore files, not just move them around and try to open them outside of Lacerte.

Just trying to think of what might be happening. The error you are getting describes a duplicate record for a file is being created, and Lacerte tries to track files uniquely, and that includes the location (full path). One reason I asked about server is the UNC path vs virtual letter has been known to be a problem with some backup vs mirror installations: "Short for Universal Naming Convention or Uniform Naming Convention, it is a PC format for specifying the location of resources on a local-area network (LAN). UNC uses the following format: \\server-name\shared-resource-pathname." That would be instead of, for example "My Z: drive is the share."