This is one of the most common questions on this site.
They are a novelty item, usually called a magician's coin, that cost $8-$10 in novelty shops and $2-$3 on eBay (plus $8.50 shipping of course). However, in some cases double-headed coins are actually genuine, not magician's coins. They need to be weighed, of course, to prove that they are not fakes.
Not a US Mint product. Two quarters have been glued together and it has no collectible value.
Heathcliff - 1980 Heathcliff's Double Big Foot 1-29 was released on: USA: January 1984
The Fonz and the Happy Days Gang - 1980 Double Jeopardy 2-5 was released on: USA: 17 October 1981
Magnum P-I- - 1980 One Picture Is Worth 7-2 was released on: USA: 8 October 1986 West Germany: 26 June 1990
It's still worth 50 cents, the dates stamped into the coin adds nothing to the value.
1980 pennies = $19.80. $19.80 is worth almost $20.
Error coins need to be seen, take it to a coin dealer. Most errors are very common and add no value.
Carabosse - 1980 was released on: USA: 1980
Dinosaur - 1980 was released on: USA: 1980
Parataxis - 1980 was released on: USA: 1980
The Racers - 1980 was released on: USA: 1980
Catch It--- If You Can - 1980 was released on: USA: 1980