Sounds you are Logging/recording login and Logoffs from yuor web page/app. If you already defined a column to be Identity, you should not specify any value for that column. Just pass the value for Scond column using Insert statement OR your ADO/ADO.NET code. SQL will take care of the column with Identity. However, if you want a value of your choice to be put into column, you Must tell SQL to SET IDENTITY INSERT ON.
Hope this helps.