"Cannot create a record in Custs2 (Custs2). The record already exists."

Custs2 is table name,i have created index(not to allow duplicates).if i try to insert the data which is not present in the table(Custs2).it is showing the  Error message "Cannot create a record in Custs2 (Custs2).
The record already exists." help me to resolve this issue.