What is the value of a double headed penny dated 1964 on one side and 1974 on the other?
This is a magician's or novelty coin, manufactured from two real pennies. While it's theoretically possible to have a double-headed coin come from the mint (though so unlikely that you may as well say it's impossible), there's no way, given the US Mint's practices, to have two sides show such a 10-year difference like that. (The same is true of the British Royal Mint, in case you're referring to a true penny, rather than a US cent.) These sell on eBay for a few dollars.
The Canadian "double-headed" cent is equal to the British One Penny. The Canadian one "cent" is equal to the British 1/2 penny (so a British penny is 2 cents or 1 double-headed cent). When first issued, the Canadian government had them struck to a standard of 100 coins to the pound of bronze rather than the the British standard of 80 coins to the pound of bronze. The first 1858 issues, however, were unpopular and…
It's worth at least a penny... But seriously, you need to provide more information to get an appraisal. What do you mean "double stamped" ? Is it a strike double? a die double? a two-headed coin? Ask a new question with more details, and perhaps a link to pictures of the coin, and we'll try to help.