Not getting expected error!

hello all - Thanks for all the help from 'fritz the blank' and 'golfDoctor' I completed a web project that includes dynamic select boxes and use of ADO to enter and extract data to a web based microsoft access 7.0 database.  Anyway I have  one loose end  involoving a unique filed.  The database is defined as follows:

Fields:(Name     -   Type    -    Size)
--------------------------------------------
           TxtNum      Double        8
           NetId         Text           50
           CktType     Text           15
           Network     Text           15
           Equip         Text           30
           Equip         TypeText    30
           Res            Text           30
           Other         Memo          0
           Date           Date/Time   8
           Time          Date/Time   8

Indexes (Name     -    Fields    -    Unique)
-------------------------------------------------
              TxtNum       +TxtNum     True

The problem is I don't know how to identify an entry as unique, once the user hits submit.  I assumed that if it tried to enter a duplicate entry it would generate an error, and I could trap the error for evaluation.  Then do something like if  err.number = x then alert user.  Instead it acts normal as if everything was fine, except that if you go back and look at the batabase the entry isn't there.  Here is the code that writes the data out:

      <%
            Dim txtNumber, txtNetID, txtCktType, txtNetwork, txtEquipList
            Dim txtEquipType, txtEquipCode, txtOther, cDate, cTime
            Dim conn, sql
            
            
            cDate = Date
            cTime = Time
            
            txtNumber = Request.QueryString("txtNumber")
            txtNetID = Request.QueryString("txtNetID")
            txtCktType = Request.QueryString("txtCktType")
            txtNetwork = Request.QueryString("txtNetwork")
            txtEquipList = Request.QueryString("txtEquipList")
            txtEquipType = Request.QueryString("txtEquipType")
            txtEquipCode = Request.QueryString("txtEquipCode")
            txtOther = Request.QueryString("txtOther")
            
            response.write("<p>&nbsp;</p>")
            response.write("<b><font face=""Arial"" size=""4"">")
            response.write("Please wait while the page updates the<br>")
            response.write("database.....")
            response.write("<p>&nbsp;</p>")
            
            set conn=Server.CreateObject("ADODB.Connection")
            conn.Provider="Microsoft.Jet.OLEDB.4.0"
            Conn.Mode = 3
            conn.Open Server.MapPath("ProActive.mdb")
            
            'set rs=Server.CreateObject("ADODB.recordset")
            'rs.Open "Select * from tblMain", conn
            
            sql = "INSERT INTO tblMain VALUES ('"
            sql = sql & txtNumber & "','"
            sql = sql & txtNetID & "','"
            sql = sql & txtCktType & "','"
            sql = sql & txtNetwork & "','"
            sql = sql & txtEquipList & "','"
            sql = sql & txtEquipType & "','"
            sql = sql & txtEquipCode & "','"
            sql = sql & txtOther & "','"
            sql = sql & cDate & "','"
            sql = sql & cTime & "')"
            
            on error resume next
            conn.Execute sql
            
            if err<>0 then
                    response.write ("An Error has occured, of the type '" & Err.description & "' ,")
                    response.write ("<br> number: " & err.Number)
            else
                    response.write ("Done.")

            end if
            
            conn.close
            
            response.redirect("http://maoc.mcilink.com/pnmc.htm")

      %>