Trust me, it has a seam. Get a powerful magnifier and look closely at the rim near the face. The machining techniques used to make these magician's coins have become quite sophisticated. Instead of gluing two halves together like an Oreo cookie, they're now made by hollowing out half of one coin, slightly reducing the diameter of the other half, and snapping them together like a jar cover.
In any case the dies used to strike genuine coins have been "keyed" for many years, so it's physically impossible for anyone at the mint to accidentally put two heads or two tails dies into the same press.
The probability of 2 coins both landing on heads or both landing on tails is 1/2 because there are 4 possible outcomes. Head, head. Head, tails. Tails, tails. Tails, heads. Tails, heads is different from heads, tails for reasons I am unsure of.
That coin would not have any collector value to them. In a US mint, it would be impossible to mint a coin that has heads or tails on both sides. So it would possibly worth a dollar or two to someone as an interesting thing to have. It's called a magician's coin and sells for a few dollars in novelty shops. They're made by cutting apart two genuine coins, then swapping and re-joining the sides.
Flip-over double-strike.
Because only the second toss matters and there's 2 sides it can land on, the chance your second toss will be tails; will still be 50%. If you want both coins to turn up as tails it would be 0,50 x 0,50 = 0,25 x 100% = 25% chance for both to be tails.
This is a novelty or magic coin that has been altered and was not produced by the mint And has no collection value at all.
If it's not a Indian head nickel, what is it? What does the coin show on both sides of it. Post new question and include any inscriptions.
A fake.
The probability of 2 coins both landing on heads or both landing on tails is 1/2 because there are 4 possible outcomes. Head, head. Head, tails. Tails, tails. Tails, heads. Tails, heads is different from heads, tails for reasons I am unsure of.
That coin would not have any collector value to them. In a US mint, it would be impossible to mint a coin that has heads or tails on both sides. So it would possibly worth a dollar or two to someone as an interesting thing to have. It's called a magician's coin and sells for a few dollars in novelty shops. They're made by cutting apart two genuine coins, then swapping and re-joining the sides.
It's a trick coin made by cutting 2 half dollars down the middle and swapping faces and tails. If it's really silver it's worth about $5 for the metal content. If it's just copper nickel it has no value because it's a damaged item.
Then it's just as fake as the ones with heads on both sides.
It's not worth much. What you have is a novelty item, not a mint error.
Flip-over double-strike.
If it has the tails image on both sides, how do you know its date is 1921? In any case if both sides are the same it's a prank coin made by joining halves of 2 genuine coins and has no numismatic value. The good news is that its silver content might be worth $13-$15. BTW, coins are minted or struck rather than "stamped"
In average condition, both coins are only worth their face value.
If it is a Mint error it may be worth a few dollars, take it to a coin dealer to see if it has any value.MoreIt's called a Jefferson nickel rather than a Monticello nickel. If by "double struck" you mean that Monticello is on both sides (which would account for the lack of a date) what you have is a novelty item called a magician's coin. There's more information at the question "What is a double headed nickel worth?"
A 2005 nickel isn't old or rare enough to be worth anything above face value, so a roll is worth $2.