Let's say that I decide I want to make an elf character who is the scion of a long line of famous wizards. Elf is obviously the ancestry I decide to go for, and I decide that the Seer elf heritage matches my concept. Among other things, Seer elf heritage gives me the ability to cast Detect Magic as an innate cantrip.
When I get to the class stage, I decide to pick Sorcerer as my class, and go with the Imperial Bloodline, because it matches my character concept. One of the automatically granted bloodline spells of the Imperial Bloodline is Detect Magic. Uh-oh.
Now, there are rules in the book that allow you to train in skills of your choice if two different sources grant you training in the same skill (like say, if you were a Ranger with Hunter background, which both automatically train you in the Survival skill). I'm not seeing any similar rules for redundant cantrip training.
Have I missed something here? Because if I haven't, this seems like something that should be addressed at some point.
For now I think you pick a different heritage if you don't want to waste a mechanical advantage.
It doesn't seem to be useful to have two Detect Magic cantrips, but it might be nice to have two Shield cantrips. Use one for a Shield Block and while it is down for 10 minutes you could use the second one.