@mlabbett -

I just tried your code and it worked fine.  The interesting thing is that I had tried @Chris_Hofer supplied code first - his worked.  I then added your code after his.

With both sets of code in there at the same time, I clicked on your link and got the dialog box.  When I closed it with the X then Chofer's dialog box came up and I had to close it as well.

So my guess is that you have two or more of these dialog boxes back to back and that is causing the issue.  Unfortunately, I do not know how to keep one from bleeding into the other one.  However, maybe what I pointed out will help you determine the fix.

edit: after posting, I went back and looked at the code - both of you used link1, I modified one of the items to link2 instead of link1 and the double box went away.