What is Longacre Doubling?

By GovMint :

Longacre doubling is a distinctive type of doubling found on coins, most notably on the Indian Head cent and Seated Liberty coins. Named after James B. Longacre, the Chief Engraver of the U.S. Mint during the mid-19th century, this effect is believed to have occurred due to the engraving process used at the time. One theory for why this doubling happened was that it resulted from a slight shift or movement of the punch used to add design elements to the die, creating a "lip" or raised edge around the design features. This doubling is not the result of a die being struck twice, as seen in true doubled die errors, but rather a characteristic of the die-making process itself.

Sources Used:

https://www.error-ref.com/longacre-s-doubling/