There are several different kinds of doubling that can be found on coins -- whether it's caused by a doubled image on the die used to stamp the coin, or caused by excessive wear or damage to the die, or caused by the coin being actually stamped twice, or caused by other reasons.
There is an excellent web page that explains the different kinds of doubling (with pictures) here : http://koinpro.tripod.com/Articles/OtherFormsOfDoubling.htm
Dan Moore The Working Man's Rare Coins http://www.workingmancoins.com
There's a picture at the Related Link.
The easy way to see one is, type in 1955 Double Die on your browser and click images.
It has doubling in liberty and in god we trust and there only 2 unowned
There is no easy way to convey in text what you have to really look for. The first thing to do is look if "COPY" or "REPLICA" is stamped on the coin, if that is on it, the coin is a replica and has no collector value. If it doesn't have that stamped in, compare its dimensions to a normal penny, if all that works, I'd take it in to a reputable coin dealer and see if he can tell you if it is a replica or not.
It looks like a penny.
It looks exactly the same as the 2010 or 2012 penny.
The US coin looks very much like a modern US penny
It is silver
like the old ones.
real copper looks like a penny
It was so different because it was double stamped. That's is about all I know. That's what makes it worth a lot more today.