Is This An Error?

That could be a legitimate "offset" printing error.
When sheets of bills are printed, the stamping image is actually on a roller, rather than on a flat printing press. The printing roll has another roll that helps squeeze the paper against the printing roll.
When the machine malfunctions and fails to send a sheet of paper through the rollers, then the image from the printing roller gets printed on the presssure roller.
When the next sheet goes through the machine, the normal image gets printed by the printing roller, but the ghost image gets printed on the backside from the ink left on the pressure roller.
The offset image will appear on the next few sheets that go through the printer leaving a dimmer and dimmer image each time, until the extra ink gets used up and the ghost images stop getting printed.
Offset print errors are valued based upon the percent of the image that is "ghost" printed. Yours looks like about a 50%. The "darkness" of the printing also affects the value. A scarce series or a higher denomination will also affect the value.
I think the earlier estimate of a $60-80 value is probably pretty accurate.
A dark 100% offset printing on a mint $20 bill would probably be worth somewhere between $200 and $400.

On the subject of using too much ink, is this an example of it?
This bill hasn't been photoshoped at all, only to scanned and resized.
The ink on the seals seem to be darker than usual. Is this common among paper money?