Don't pharmacists in the U.S. look at drug interactions before they issue new meds?
Yes, but patients often don't tell each doctor what their other doctors have prescribed for them. And, as Foxfyre said, a lot of people don't use the same pharmacy consistently. They just use whichever is most convenient that day. It's not uncommon for someone to have prescriptions on file at several different places.
Our family, for instance, has prescriptions at three different pharmacies (one close to home, one close to Hubby's work, and one close to the doctor's office for emergencies) as well as our insurance company's mail-in/internet pharmacy for maintenance meds.
Normally, our insurance company checks for possible interactions & duplications when we present our card (or account number) for prescription benefits. Once we are given a 30-day supply of something, they won't approve a refill for 30 days. But people who are turned down can easily circumvent this. They can always go to another pharmacy and pay cash.
There are lots of ways to beat the system if one wants more drugs.