Hi,
After any event, if you refresh page then it prompt for it and if we allow it it redo last event.
Now in your situation you are inserting values in db. So on refreshing page again your db insert query fires and it insert value in db.
Now you have two ways to over come this.
1. After inserting db value redirect to same page.
e.g.
Response.Redirect(Request.Url.AbsoluteUri);
2. In your db query, first check for value, if same value exist stop inserting in db.
Hope it helps you.
Thanks.