Thanks for the query.
Some women may have low HCG levels in early pregnancy
that may affect the growth of the baby.
As the sac was not there in your first scan, the HCG levels were at the borderline and as you are having previous history of miscarriage
your doctor might have given you on HCG injection
and duphaston(which is also a progesterone) tablets will help in maintenance of pregnancy and prevent miscarriage.
May be your doctor thought that continuation of the treatment may be needed and that would be beneficial to you, so he might have told to continue the pills.
Don't worry usually no harm will occur with continuation of these medicines.