我的代码为什么一刷新网页就向数据库增加空信息啊?

<%
dim tit,comm
tit=request.form("newstitle")
comm=request.form("newscomment")
set rs=server.createobject("ADODB.Recordset")
sql="select * from news_comment"
rs.open sql,conn,1,3
rs.addnew
rs("title")=tit
rs("comment")=comm
rs("posttime")=now()
rs.update
rs.close
set rs=nothing
response.write "<script>alert('提交成功！')</script>"
response.end
%>