How much is a penny worth with a doubled side momument on both sides of a penny?

If the memorial on the side with Lincoln's head is sunken instead of raised, and all the writing is backwards, there are a couple of possibilities.

One is that someone took two coins, and hammered or pressed them together. Since it happened outside the mint, it's just worth face value.

Another possibility is that it's what's called a "brockage" error. If a coin doesn't get ejected after it's struck, and another blank is fed into the press, the second blank will get an image from the struck coin on one side. That seems really unlikely in this case, because the original coin would not only have to have been still in the coining chamber, but would have had to flip over before this coin was struck. So I'd be 99% sure that you've got a coin that someone damaged with another coin--although you can always take it to a coin dealer (who can see the coin up close) to find out for sure.

If the image of the memorial on the "heads" side is raised up, then it may be that you've got a coin that was struck, flipped over while in the coining chamber, and then struck again. If that's what you have, again you'll want to take it to a coin dealer to get it looked at.

The problem with trying to determine how much a coin is worth on-line (especially with errors, which tend to be different every time) is that you really need to have the coin "in hand" to look at it closely and see exactly what you've got.