I'm using DTRegister 2.6.8a and I'm still getting a large number of duplicate registrations (currently about 50 in 750). They aren't appearing in the back-end but they are in the exported csv file which we are using to print up name badges. Also, group registrations are not receiving a registration number.
You need to first update to the latest build of DT Register... then if you still have an issue, log a support ticket with more detail like payment method used on those records, etc. We are not receiving any other reports like this for 2.6.8.