Try reading the instructions. https://support.mozilla.org/en-US/kb/using-lightning-calendar-add-on

Thanks Airmail... but try reading my question. The instructions don't answer it.

Well I follow the instructions and have never had a duplicate calendar.

Was it local or online?

It's a local calendar.

Judging by the two prompt responses I've received (thank you both) it must be something I'm doing wrong, rather than a Calendar glitch, so I'm going to close the question and use the Windows calendar as a quick-solution, additional calendar. I thank you for your time.

OK, I don't have Thunderbird here in front of me right now, but I'll look tomorrow at what happens when adding a second local calendar.

Most of the queries we get here are about online calendars, so collectively I suspect we have little experience with multiple local calendars.

However, unless you're prepared to enable and disable the two as required, I'm not sure I can see that there would be any less clutter. You could of course have them in different colours. You'll also have to pay attention to which one is active when adding an event or task.

New calendars are just overlays on the calendar form. Use the checkbaxes on the left to turn on and off the ones you do or do not want to see.

Thanks Zenos - if there is anything you can suggest tomorrow so that I can have two local calendars containing different events, I would be grateful since I would rather use the additional calendar in Thunderbird than use the Windows one. I understand about enabling/disabling... it will be OK for me.

LadyPenelope said

Thanks Airmail... but try reading my question. The instructions don't answer it.

I can read just fine thank you. I seem to have figured out the calendar fine using those instructions.