Crab,
My understanding of a "Duplex" bearing is that it's actually two ball (or roller) bearings sitting side by side in one case. They are used in various applications such as vehicle wheel bearings. They provide an alternative to having two separate bearings with a space between them and are used where lateral "rocking" or "run-out" needs to be minimised and also provide load sharing between the two races. They need to be carefully installed to avoid damage, more so than simple ball or roller bearings. If they are knocked in too hard (such as to overcome too tight an interference fit), they can suffer damage.
Are you sure you're not mixing this up with what is known as a "linear" bearing? A car gear change assembly I rebuilt had a small linear bearing in the housing. It allows the gear linkage's horizontal shaft to slide fore and aft (as when going from 1st to 2nd or 3rd to 4th) and rotate from side to side, as when going from 2nd to 3rd. These have rows of small ball bearings sitting in longitudinal slots in a tubular, centre cage which holds them in alignment. I ditched the rusted out one in my gear change and because I had great difficulty in finding a suitable replacement I fitted a plain, sintered bronze bush instead.