Set/Add on list params

[aolofsson]

I have made this mistake a few times now, (with option, ydir), and each time it's a few minutes gone. I know we have gone back and forth on this, but I would think that doing clearing out a list by using set list param that has already been set, should be a rare occurrence for the user? It would be great to get some feedback along the lines of "hey, did you really mean to do this?".

[nmoroze]

Makes sense, we could make this a warning (maybe with a flag to quiet it as well). I think we'll need to fix #1146 first for this to make sense though, otherwise we'll get a warning on everything that has a default value.