Retina Banner

[Azhiya]

Does the user load only a Retina double sized banner – is the standard banner auto created by optimizing the file to 50% dimensions automatically during load?

OR does the client load standard size creative and Retina double size creative to same campaign?

Yes, You can upload standard size creative and Retina double size creative to same campaign.
Retina banners can create using Retina Double sized banner type.
You should upload all possible standard sizes of  Retina double size creative.