Revit doesn't like opening a linked file in the same session, without unloading the link in the current model first, but it won't mind doing so if you open a second session of Revit. Revit uses separate memory allocation for each session. That means it isn't possible to use Copy to Clipboard with Paste Aligned when we are using two sessions. If you let Revit unload the link instead you won't see any changes in the host project until you save, close and reload the linked file. A good many users regard that cycle of steps to be annoying.
When we enable Worksets we have a Central File but work in a Local File. If all the project files we use have enabled Worksets then when we open any of the Linked Files we are creating a new Local File. Here's the tricky part...technically that's not the SAME file we Linked. The Linked File is (should be) based on the Central File (it's name and location)...the Central File is linked, not our Local File.
Yes this means you can now open a linked file in the same session of Revit. You can make changes in either file and use Synchronize and Modify Settings to store the changes in the Central File(s).
Now before you get too excited, you still have to use Reload on the Linked File that's been changed. That's not really any different than having another user making changes to the Linked File and having to use Reload to see their changes. It does make it easier to go back and forth between models quickly; eliminates the open/close part. Eventually you have to use Reload to see any changes regardless.
If you are a sole user and still intimidated by Worksets; just remember you only have to have one Workset for it to be enabled, for Revit to work. Revit creates two default Worksets for us to use (Shared Levels and Grids and Workset 1) but we don't have to be too concerned with assigning elements to any but Workset 1. That's assuming we don't really need Worksets for its fundamental purpose; allowing concurrent access to the same data by more than one person.
Something to consider if open/closing and unloading/reloading links is annoying.
Oh, I should mention that this starts to disintegrate if you are opening more than two files that are inter-related, linked into each other. For example, imagine a Host Model, Linked Model 01 and Linked Model 02. The Host Model has linked both of the linked models. If Linked Model 01 is also linked into Linked Model 02 and we then open both of them as well as the Host Model we will encounter this kind of message when we make changes to Linked Model 01 and then attempt to reload it in the Host Model.
The file in question is also present in the other open Linked Model and that is what Revit is objecting to. We'll also find that the file is unloaded automatically. We'll have to close the other file that it is visible in before we can successfully reload it. As such my habit is to limit my use of this technique to two open files at a time.