Hi Danie,
If I understand your questions correctly, the MDG check (Duplicate) is not working for the records created/ upload/ existing in ECC system (using T.C).
In case of records created by CR from MDG can be seen during Duplicate check.
Reason could be for records created/ loaded in back end ERP system, certain fields could have been mandatory or missing compared to Fields in MDG Change Request. (e.x. Authorization Grp)
Please compare the records, this could be the reason why the records created in backend are not coming up during Duplicate search.
BR
Srihari