Actually, technically the FIRST was the Italian Villar-perosa of WW1 fame. Tw blowback action side-by-side with top-mount mags and a bipod! Not really an SMG in the conventional sense, but that is why I said TECHNICALLY.
After it was realized that the Villar-Perosa was useless as a GPMG (it was originally designed as such, for mountain troops), most of them were stripped down for re-build as OVPs; an OVP is just one half of a Villar-Perosa placed into a wood stock.