Scriptures denying Advaita started appearing around the time of Ramanuja. But before that - do Vedas, Upanishads,Brahmanas, Aranyakas, epics and Puranas all have some passages in them that are advaitic?
Yes, there are advaitic passages in scriptures. I am giving below one example.
Yayati answered, 'The wise, with the help of the Vedas and of Knowledge, having ascertained the visible universe to be illusory, instantly realises the Supreme Spirit as the sole existent independent essence. While that they that devote themselves to Yoga meditation take time to acquire the same knowledge, for it is by practice alone that these later divest themselves of the consciousness of quality.'
Mahabharata Adi Parva, Section XCII
Adi Sankaracharya did not invent Advaita although it was most probably not the most popular Vedanta school before him.