A travelling merchant arrived while I was upgrading the Tier 1 Trading Post to the Tier 2 Trading Centre.
The Trading Centre upgrade finished while the merchant was still present, and I received a notification a new travelling merchant had arrived. Trader 2 is a complete duplicate of Trader 1.
Screenshots of Trader 1 and Trader 2 after the upgrade for the Trading Centre completed
Edit: I went back to a save before I upgraded and reproduced the bug. Upgrade the Trading Post; there are enough resources but leave it unprioritized. A merchant will arrive at around 50 planks and 20 bricks needed to complete. Once the Trading Post completes a duplicate notification will come through alongside the Trading Centre showing two of the same trader.