This can happen for a few reasons and if the seller follows the 3 rules to win the buy box (FBA, New, Match Price) they will get a rotation in the buy box for each listing they have that matches the buy box criteria. This means a seller can get double, or even up to 5X or more, rotation of the buy box! This can be huge for a seller because it can greatly increase sales. 

How can one seller have their name listed as a Prime seller twice on a listing? Does this mean the seller gets two shares or two rotations of the buy box?

So, why do some sellers have more than 1 listing on an ASIN and how can I do that? 

The main reason they have multiple listings is because they have sent in the same ASIN under different MSKUs6 or FNSKUs6. This usually means they have sent some inventory in using manufacturer barcodes and then sent in additional inventory using Amazon barcodes. This mean the created two separate listing in seller central and then selected different barcode rules for each listing. 

Because Amazon commingles Manufacturer bar code items and keeps items with your unique Amazon barcodes separate from other items with the same ASIN, Amazon displays each of these barcodes by themselves on the same listing. 

This means sellers can get more than one rotation in the buy box! 

Other reasons sellers can show up twice on a listing is FBM, Seller Fulfilled Prime, Different Amazon Barcode, different MSKUs, etc.