1968 S Doubled Die Obverse Penny ?

Medium date ? Or Machine Doubling?

Looks like MD to me as well.

A noted already it is Machine Doubling. Machine Doubling is really common on the 1968-1972 cents. In 1973 they tried something different that year with the strike, (thus the tall rims on the 1973 cents) that fixed the issue a bit, but we still see it on even the current coins. Which having a lower profile, allows this to continue, but the years I mentioned earlier had a lot of Machine Doubling. Even the big DDO on the 1969-S, most examples show Machine Doubling on them:

Note the hub doubling on the dates? Then note the yellow arrows. Note it only affected these in the same direction? That is what Machine Doubling does. It affects the outside devices, flattening that area (reducing the devices) in a direction. So on a coin that has Machine Doubling, the direction is across the whole coin in a direction, not affecting the other areas of the single devices.

Yes. It is a doubled die, with Machine Doubling. The MD doesn't cancel out it being a doubled die.

The 1968 was listed as a doubled die, but later determined that it is master die doubling:
http://doubleddie.com/814639.html
Note on the description that all mints were included in the master die doubling?

This is not Machine Doubling. This us doubling that was padded down to the working hubs and the working dies from the Master Die. (So as many as 50% of the cents that year from all three mints could have been affected. If caught, the earlier, the the issue could be been cut short. Did it? I don't know)
While the hub doubling is on the edge of the devices, note that is doesn't create a flat area on the devices, like Machine Doubling does. Also it doesn't push the metal upwards like Machine Doubling can.

CoopHome: 1968 cents do have master die doubling on them