I've seen the thread about two people finding a bomb from world war 1 and the question just popped in my mind:
So... WWII started roughly 25 years after WWI ended, and given we are still finding undetonated bombs from the first world conflict 100 years after, I assume that 25 years after it was a more common occurrence.
Were the dangers of those old battlefields a known risk? Did the leaders, in its strategies, avoid the places where battles had taken place in WWI? Was it for respect, "security" or just because the change in how the war was fought between both conflicts made the old battlefields strategically irrelevant?
Certainly the dangers of these old battlefields were well know. Millions of shells were fired over the course of WWI, a great many of which were duds. In areas of intense battle, such as Ypres and Verdun, there were (and still are) quarantined areas that are considered too dangerous for the general public to enter due to the sheer magnitude of undetonated ordinance.
That said, the main reason you don't see "repeat battles" in WWII is strategic and geographic differences. WWI's battles of note were fought with trench warfare mainly along or near the French-German border. However, WWII was fought with a focus on being highly mobile and flexible, with the European Theater spanning from the Mediterranean all the way up to the Baltic. It's also worth noting that Nazi Germany's Blitzkrieg swallowed up France shortly into WWII, before large scale battles could be waged at these old battlefields.
Battles in WWII just didn't fall in areas where battles from WWI occurred. Certainly tactics and the sheer geographic magnitude played a role in that fact, but I wouldn't included fear of undetonated ordinance as one of those reasons.
I just want to add to this that finding explosives happens every day here in Belgium, esspecialy here around the yser river and ypres. Everyday farmers dig them up and place them alongside the road for the army EOD to pick them up. I can't give any sources at the moment because I am typing this on my phone, but I am in the Belgian army and we here a lot of crazy stuff from the EOD teams.