I have 2 very simple (1 room) test cells, each has a "load" type door. Whatever I try, as soon as I "connect" the doors using the Teleport tab I get [link removed] and a "Duplicate000" cell when reloading the cells.

Bound to be a noob error on my part, but I'm out of ideas. Anyone give me a clue as to what I'm doing wrong please?

If anyone wants the .esp files to look at they are [link removed] though they're pretty boring - they are test cells after all

From the CK's point of view you load two Identical cells, the CK tries to fix that by creating a new cell.

Your second ESP file is a Windows copy of the first one, there is no chance that all FormIDs are identical otherwise.
Also, it seems that you created the first of those ESPs with two other ESP files loaded in the CK (FormIDs start with 03 but Skyrim.esm is the only master). This is not the standard modding practice, and as you've seen, ESP files can't interact with each other, at least not without considerable effort. Working with several interacting stand-alone ESPs is a very advanced kind of modding and generally more trouble than it's worth.

I assume you want to make one mod, not several mods that interact with each other. In this case you need to use the "Set as Active File" button in the File/Data menu to tell the CK that you want to continue working on your existing ESP instead of creating a new one and using the loaded plugin(s) as template(s).

To create a copy of a room, do it in the CK (in the cell view window right-click / "Duplicate Cell")

No - I'm sorry but they are NOT copies. Each was created, from scratch, in the CK and saved individually. Both were started by duplicating the aaaMarkers cell (as shown in the "Lorkir's Tomb" tutorial) as I don't know any other way to create a "new" cell. Nothing was copied from one to the other at any point.
I'm also pretty certain that NO other mods were loaded when the first was created. It is possible, however, that the first was still loaded (but not active) when I created the second.
These cells are precisely what they say they are - cells for testing things - in this case the doors. They will not, directly, form part of a mod.

No matter how you managed to create 03xxx Formids that are identical in two files:

You can't link doors in different ESPs.

Fair enough - I'll have to try something else then.