Urgent help needed- miscarriage then pregnant again

Hey guys, posting for a friend here. She unfortunately had a miscarriage on the 1st of September, it was unknown how many’s weeks she was but her HCG was 100 and clear blue digital was showing 2-3 weeks. She began her pill once again and missed a few days here and there. Fast forward to today she went to get the coil fitted and they done a pregnancy test and surprise she was pregnant again!! She done a clear blue digital afterwards and the result showed 3 weeks plus. So my question how is this possible? Is this a new pregnancy? Or maybe she didn’t properly miscarry in September ( even though her HCG went down to 49. Has anyone an explanation for this? Thanks in advance x