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.
You might have a flip-over double-strike which would bring several dollars from an error collector, but you should be able to see at least a trace of heads and tails on both sides. If the tails image that is on the heads side is reversed, it is possible that someone laid another coin on top of yours and hit it with a hammer. If that's what happened you just have a damaged coin. Someone familiar with minting errors would have to examine it to be sure.
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.
All error coins need to be seen. Take it to a coin dealer in your area.
Well, it should have rims on both sides and be properly struck, so I'd say it's worth 1 cent. If the rims seem higher than normal, it may be an early strike from a new set of dies. Coin Market lists a retail value of 25 cents for either 1978 P or 1978 D Lincoln Cents in Premium Uncirculated condition.
A fake.
It's a trick or magic coin and has little or no value.
A two-headed penny.
If this coin is indeed half struck on both sides, it could be worth a lot of money. Advise you take the coin to a coin dealer to have it appraised.
Both are only face value.
$14,500
The head's side of the penny is heavier because, the indentions on the head's side happens to make it slight heavier but I have tested this project out and it is not a myth it is head's.
Then it's just as fake as the ones with heads on both sides.
You might have a flip-over double-strike which would bring several dollars from an error collector, but you should be able to see at least a trace of heads and tails on both sides. If the tails image that is on the heads side is reversed, it is possible that someone laid another coin on top of yours and hit it with a hammer. If that's what happened you just have a damaged coin. Someone familiar with minting errors would have to examine it to be sure.
Flip-over double-strike.
Nothing- return it to the store where you got it. These have been showing up in penny rolls. it is play money.
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"