Ideas
After updating to the recent patch, I tried visiting Henford to see the Creature Keeper.
Before the patch, there was a new random sim instead of Michael Bell. After the patch, Michael will appear every time, both modded and unmodded save.
I'm unsure if there is any change in this part, as it wasn't mentioned as "fixed". However, I haven't tested duplicating NPC, which needs more time staying on the lot. So, if anyone has an update about this bug, please share your results here.
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Also, last time I mentioned sims4me's Keep Premades - Townie Overhaul Less Fame mod. But I forgot that I downloaded SimVasion's Villager NPC Roles Fix before testing as well. So, I don't know what mod can fix. But these mods don't conflict each other when checking mod conflict. (that's whay I don't know that I already have 2 bugfix mods)
Currently, I use sims4me's one because it's not just for Henford NPC but also other NPCs in other EP/GP, too. So, if you try to find the mod to fix, try 2 mods I mentioned.
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Or if you don't have duplicate NPC sim bugs anymore, please report here, too.