Hello Alok, 

This is a very general question but the document verification is one of the most important procedure during admission in any college or university. 

If you have appeared in first round of counselling, then you must have had all the documents you needed there. So, in second round as well you need to give all the required documents asked by any institutions at the time of counselling. Whether it may be 1st or 2nd or 3rd round of counselling you must have all the documents mentioned at the website of the board of counselling. 

No you don't have to go for document verification again if your documents were verified during the first counselling process but you do need to go to the document verification centre (the same centre where you went for document verification) again to deposit the security fees demand draft, if you weren't alloted any seat during first counselling.

Source : https://upneet.gov.in/PDFFile/Ad