At Gauhati University, you can appear for your back papers in alternate semesters. So, you can appear for your 1st year back papers in your 3rd and 5th semesters and not in your 2nd and 4th semesters. 

You can take your back paper as many times as you want. However, you need to clear all your back papers within 6 years of your admission. Also, you can’t appear in your 6th-semester examination, until you have cleared all the papers for your 1st and 2nd semesters including all your back papers.