1986 P Lincoln Cent

It's die wear. This happens a lot on early zinc Lincolns.

Something that will help you along your journey, and this example is a good one to use.

Look at all the areas around where you are looking at something you don't completely understand. Observing the entire area may help solve the puzzle.

This coin shows a LOT of rippling throughout that area of the design. The rippling you saw as doubling is only a piece of a larger picture. Because hub doubling (doubled dies) are confined to the devices (letters and numbers) themselves, the condition of the field around the devices doesn't matter...but on this coin, it's obvious that the condition of the field around the devices definitely has something to do with the rippling of the number.

So, since the rippling in the field and the number are related, it cannot be a doubled die. Subsequently, the only thing likely to affect BOTH would be wear on the die. Breaking down the appearance of what you have into a mechanical way of thinking helps a lot in determining the answers to your own questions.

'Flat field doubling' is a term some people use, and I believe it is synonymous with Machine Doubling, and not one of the particular causes of Machine Doubling. In other words, I believe it is used for nearly anything that's doubling but not a doubled die.