The world I'm currently playing on has 4 Jens, 3 Rekts, 1 Joel, and 1 Bob. Sadly no Hughs. Depending on the size of your world, there's bound to be duplicates because there's a trader in every town/city, but there are only so many different trader characters.
As others have stated, the reputation you build with one trader will carry over to all duplicates of that trader. It's best to think of all duplicates as the same exact character, but they just have multiple locations. Regardless of what trader you go to and whether or not there are duplicates, quests only refresh overnight, so if you complete a tier, you won't actually see any quests for the next tier until the following day. It's highly likely you visited your Jens on different days which would explain why they had different tiers.