変数 Itemtext は既に宣言されています。なぜこのエラー??
Try
conn = New SqlConnection(connString)
conn.Open()
Dim i As Integer = 0
Using cmd As New SqlCommand()
For Each c In item
cmd.Connection = conn
cmd.CommandType = CommandType.Text
cmd.CommandText = "INSERT INTO tblItems(Item,Numberof) Values (@Itemtext,@Numberof)"
cmd.Parameters.AddWithValue("@Itemtext", c)
cmd.Parameters.AddWithValue("@Numberof", num(i))
cmd.ExecuteNonQuery()
i = i + 1
Next
End Using