1968 S Lincoln Pennie. D On "S" And "Liberty"

Dude, you gotta wait more than 25 minutes before bumping your thread. Please. Also, as long as I'm being a crusty old crab, just a suggestion that your pics would be significantly easier to look at if the design elements were horizontal rather than at an odd angle.

I don't see any doubling on LIBERTY, although there may be a very small die crack between the first two letters. You could have a re-punched mintmark. Have you looked to see if this is one already identified?

Certainly looks like something happening with the S. Let us know what you find out.

The mint mark is showing an issue with a damaged punch. How can you tell if it was an issue thi year? Looking at coins from that year. Even different denominations used the same mint mark punch until it is replaced. On the 'S' coins, the devices break down most often above the lower serif area.
Note on the lower images how the inside parts of the mint mark fill in from die damage.

There was was an issue with the 1968-72cents with a very high number of MD on the devices. But MD doesn't show spreading, but opposite, it reduces the devices even smaller and looks lower than the normal device. They are flat, step like and the normal contours are flattened. I don't see a premium for your coin. But it is a copper coin if you are saving them.