Action items might need to be part of more than one list.

For example: you can copy an action item from a meeting note into your personal todo list.

And if the item is linked, the item will be synced between the lists. So if you check off an action item in 1 place, then it will check off in the other. 

And linked action items will only appear once in the action items section despite being in two places.

By hovering over any action item, you see three vertical dots at the end of the line (the far right). By clicking this, you will see an expanded menu:

Select the option to 'Copy-to'

Which will open this modal:

There are a few things that need to be set here.

First, it's where you would like the action items to be copied to. Type in the name of the meeting note/note series and/or select from the drop down.

Next, choose whether or not you want this to be a linked copy.

If it's linked, there will be a purple link icon on the left of the action items. And if you check off the item in one place, this will be reflected in the other place.

If it's not linked, then the item will be more like copy/paste where you will have two copies of the same action item that are independent from the other.

As an example, I copied this action item from "My Daily Planner" into the "Leadership All Hands". When it is linked, I can click on the action item and go to the other note that it is linked to: