XnView Software

So perhaps the 'Only one instance' option

Almost........when I use this option a tab which is already open gets replaced by another picture (tab) which I open from outside Xnview (Windows explorer).

But you've put me on the right track!!
Disabling the option via Tools --> Options --> View ...

I've been searching for an hour in this forum for the answer on how to get
Xnview to work in a single window mode .

If the answer to my question is still already out there, I apologize!

No matter what I open after Xnview is already opened, I want it to open in
the existing Xnview window in ...