Check out some photos of repunched mint marks. They have very easy tell tale signs to look out for. Some are difficult to chase just like anything else however most are easily distinguishable. You should also check out die chips and such. Will help you understand the difference.
I don't think there even is an S over D 1968 Lincoln. Here is a list from the LCR and some interesting text below. 1944 d/s omm 11944 d/s omm 21946 s/d omm 11951 d/s omm 11951 d/s omm 21952 d/s omm 11956 d/s omm 1 wdmm 1 (dual mintmark)1956 d/d/s omm 2 The above list represents the most commonly accepted over mintmarks (OMM's) in the Lincoln Cent Series. It may seem strange to you that the mint would change its mind after punching a mintmark, especially if you think that the S mintmark is punched at the San Fransisco Mint and the D mintmark is punched at the Denver mint. The truth is that both the S and D mintmarks are punched at the Philadelphia mint where all dies are prepared. So a mint employee could easily change a D to an S for a variety of reasons before the dies are shipped to their respective mints. What about the 1980 D/S cent? The 1980 D/S omm 1 was debunked in 2006. Die Variety News had an article proving that what looked like an S was actually die gouges in the field. The article was Vol. No 2. July/Aug 2006 written by Billy Crawford. Also, ANACS no longer certifies this variety. Some collectors and dealers are not aware of this. Investing in this variety is not recommended.
