Add layers to legend multiple times

[IlkaIllers1]

Very often, I have to duplicate layers and/or work with dummy layers because you can only add them once to the legend. It would be helpful to be able to add them several times:

1. In order to use the "Only use layers from this heading" more effectively - sometimes I want layers from two or more headings, but with other layers in between them

2. In order to move the description to start below the label if the legend field is quite narrow

3. Lot of other reasons!