Use distinct.
string stqr = "select distinct c.Cand_Id,c.Candidature_Course,c.Candidature_Photo,c.Candidature_Stud_Nm,c.Candidature_Stud_Lnm,c.Candidature_Stud_Email,c.Candidature_Stud_Contact,c.Candidte_Int_job,c.Candidature_Stud_Resume from Candidature_Inst_Details c,Institute_Details i where c.User_Name = i.User_Name and c.Candidature_Course = '" + DropDownList1.SelectedValue + "' and c.Institute_branch = '" + DropDownList3.SelectedValue + "'and c.Candidte_Int_job='Job' and c.Cand_Id NOT IN(select Cand_Id from cand_post_details where Posting_Id='" + p1 + "')";
I dont understand why you have table"Institute_Details" in from clause when you are not using it in select and where clause. if you not using at both places remove it and remove
c.User_Name = i.User_Name