In this context, unusually means extremely. Unusual is not a negative, so the concept of a double negative does not apply.Unusually messy = you cannot see your bedroom floor due to the things strewn all over it. There might be some unfinished food under there somewhere, too…Messy = yeah, there’s a bunch of stuff laying around, but you can see the floor and tell where the furniture is.Neat = pretty much everything is put where it belongsUnusually neat = absolutely everything has a place and you would find it annoying and distracting to share a space with someone who was not the same way.