For decades, Germans have had laws and enforcement of a public ID card. You can go and read up on this via Article Four at the German Justice Ministry.
It's a simplistic law. Upon reaching 16 years old....you are mandated to have a national ID card.
If asked to present the ID, by any government authority (the cops, the Justice Department, the vote clerks, the customs people, the grocery clerk noting you are buying smokes or beer, or the city hall clerks)....then you must present the ID.
The Germans who are waivered out of this? It's a very small group. If you are a permanent resident of some mental facility or old folks home....you don't have to keep the card. If you are living in a situation with a guardian presiding over you.....you don't have to carry the card. If you have some kind of serious disability where you have to have someone travel with you.....you don't have to carry the card. Otherwise, it's something that Germans maintain on their person 24 hours a day.
Odds of having multiple cards? Zero. At least that's the strong belief of the public.
What info exists on the card? Your full name (to include your name before your marriage), you doctoral degree (don't ask....doctors seem to want everyone to know their status), date of birth with your place of birth, a photograph (something recent), a signature, you height but oddly NOT your weight, your eye color, your address, citizenship, a serial number, and your religious name or 'Hollywood' name (this covers Cardinal Joe or Gina-Gina-Gina).
Cards are valid for a ten-year period....although if you are under 24 years old....it's for six years at a time.
Effective outside of Germany? Well....you can use this to travel to any EU country. If you go beyond the EU country scheme.....you need a real passport.
Cost of the card? It's roughly 30 Euro.
Ninety-nine percent of Germans will go through their life without any problems with this ID card business. Typically, the biggest of the violations are failing to present the ID if requested, or failing to update the card (like you got married and use another name).
So why does this work in Germany but not in the US? There is this odd perception of governmental control in the US and that duplication would be a massive problem. When you look at the one-ID system, it concretes down the German voting system and ensures virtually no fraud makes it into the system.