I have found this dime listed from S....P.....but I have saw no D's. So either there were no 1968 D DDO's or the 8 in these dimes look like a DDO?
There were 480 million 1968-D Roosevelt Dimes minted according to the 2014 Redbook. Anything further regarding the possibility of a DDO requires pictures.
Something definitely does look weird with the 8. In my unprofessional and inexperienced opinion, I would suggest machine doubling, but since I'm not an expert, I would wait until someone who is comes along.
Dimes are a whole new world. The DDO for the S and P mints look just like this, the smashed 8 is the marker for this variety. I have looked into them a little bit.