iPhone model 3M: Original or Duplicate?

That usually indicates that the unit has been refurbished by someone other than Apple, at a facility approved by Apple. For warranty information, you will need to contact the retailer.

Is this device originally from Apple?

Yes it is. But, there is no way to know how old the device might be, or exactly what was done to the iPhone during the refurbishment process.

Does the seller offer a one year warranty on the iPhone?

Does the seller provide you with a 14 day money back guarantee, if you discover something unexpected about the phone?

If the answers to the two questions above are "no", my advice would be to not purchase the phone.

That's up to you. Personally, I wouldn't buy it. If I purchased a refurbished iPhone, I would ONLY buy it from the Apple Refurbished Store, where I knew it came with only authentic Apple parts and with a one year warranty from Apple directly.

The model number simply indicates the source of the device, but no one here can tell you the condition of the device. If parts had been replaced, I can only assume they used genuine parts.