I need to check information the user has entered on a page against what I have in my database to see if he is entering duplicate data. It is up to him if he wants to proceed or not. I just need to alert him.

The problem is I don't own the button that he clicks so I can't hook up a javascript confirm to the button click. I am running within vendor code. I have an "override PageLoad()" procedure where I can test what page they're currently on. If they are on page "EditForm.aspx" and would normally navigate to "UserFinish.aspx" without any custom code of mine "interfering", I want to test what they've entered on EditForm.aspx and present them with a popup if they've entered duplicate info, and if they click OK button then proceed to UserFinish but if they click Cancel then stay on EditForm.

I've googled and found some things that might work, but it's usually followed by a caveat saying "do the following, but I don't recommend it", so I just wanted to ask for a way that is recommended. Thanks.