The situation brings to mind the first couple of lines from Bonnie Banks O' Loch Lomond. Your supplier definitely "took the high road". As greedyboy said, what they did voluntarily was unusual for many suppliers, foreign or domestic. They should be applauded.
No, you get to chose which road you'll take. If you are also going to take the high road, you will contact them and let them know what has happened. From that point there are 4 things that can happen that will keep you on the high road.
If the value is not too great they may simply tell you to keep it - free.
They may ask you to return it at their expense - the most equitable, in my view.
They may ask you to return it at your expense - I'd have to give this some consideration.
They may ask you to pay for it and keep it - as mentioned, this may or may not fit your needs.
Or, there is the low road -
"What second package, I never got a second package"
"Screw you, I got it and I'm keeping it."
"I'm not paying to correct your problem."
Etc.
Keep in mind these people gave you exceptional service by sending the second shipment before you even inquired. They deserve every consideration in return.
If you were buying once for yourself, that's one thing. But if you use them, or ar considering using the as a supplier there is nothing you could do to cement a positive relationship than to let them know today what has happened. And then if they used a courier type shipper I'd suggest to them that they issue what UPS calls a "call tag" where the UPS comes with a prepared label and receipt and picks up the package from you at no cost to you. I'm only familiar with UPS with this practice, but I'm sure the others offer it as well.
And remember - what goes around, comes around. I think some call it The Golden Rule.