I'm still astounded that they actually exist. Given the way that the dies fit the press, how do these happen, @GDJMSP ?
Strike issues. Clearly something went horribly wrong for that to exist but the grade is kind of meaningless, it's more just saying it is Uncirculated and becomes a ranking system if another ever popped up
Because dies aren't/weren't made the way you seem to think they are Mike. They are all round. Now I don't have pictures of a lot of old dies, both obv and rev, but even hundreds of years ago when they still hammered coins they were round. These should give you a representation at least. Here's an old hammer die. Here's an old hammer die and an anvil die. Here's a picture of a milled die from the early 1900's. Here is a picture from the mint website of the progression of how modern dies are made, from bar stock to completed dies.
"slightly weakly struck" To produce a train-wreck like that, I can imagine almost anything. Two obverse dies, a planchet stuck in a piece of pipe, and a sledgehammer, while somebody else stood watch and/or took care of the security cameras...
And look what just tonight turned up on eBay: https://www.ebay.com/itm/TWO-HEADED...2-Obverse-Dies-PCGS-MS-65-UNIQUE/273180885732
As you can see in the second photo the circumference is slightly smaller than the first. It has been machined down, and the first coin hollowed out, and the second coin placed inside, to create a 2 headed coin. If you look for a seam, or try to pry them apart you will probably see that there were 2 coins hollowed/cut in half to make this.
The obverse and reverse dies have different attachment hardware to prevent that very thing from happening. The odds therefore are zero.
Not quite zero. (Link goes to story with video, starring our very own @Fred Weinberg.) But close enough to zero.
Nice. But it isn’t really a “legitimate” error: “Something that shouldn’t happen, but apparently did at the hands of some enterprising mint workers.”
If it came out of Mint equipment, it's apparently "legitimate enough" for the TPGs, and for enough collectors to constitute a market. I don't much like it either, but I'm not sure what to do about it (or, for that matter, those 1913 V-nickels).