I have a small and simple pure HTML website here, which contains an advert in a
table on the left hand side of every page in the site. I only ever checked that it worked on Firefox on a PC. But just now I have been alerted to the fact that on the front page, the advert appears in two locations in both Chrome, Internet Explorer and Firefox for Mac. Even more strangely, the double-advert looks fine (i.e. just appears once) on every other page on the site, regardless of the browser - even though the code for the ad is identical.
I'm completely baffled.