Posted by LordOfBikes on Dec 28, 2020; 9:24am
URL: https://forum.librecad.org/Layer-list-tp5719931p5719936.html

There are already some discussions about this here and on github.
The problem is, that the LibreCAD undo/redo system keeps layer names on undo, in case you use redo.
So I assume you added a layer, then used undo, what removed the layer from the list and then tried to add a layer with the same name again. This doesn't work.
But when you close the file and reopen it, the undo/redo system for the file is reset and it is possible to add a layer with the same name as before.