Store brands posted double-digit sales increases across US supermarkets, discounters and drug stores as shoppers stocked up on products during the first stages of the coronavirus pandemic, according to new research from Nielson.   First quarter dollar sales of private label products across all retail outlets compared to the year before climbed nearly 15% during the first quarter, up $4.9 billion. Unit volume increased nearly 13%, representing a gain of around 1.5 billion products sold. Total d...