SQL's biggest design flaw was rejecting the relational model, which leads to the join problems the earlier comment talks about.

The second biggest flaw, which isn't really about SQL itself but in implementation, was attaching SQL to the network where latency starts to become a problem. That requires hacks to work around. This is likely this is why you're seeing data duplication — trading all that duplication to avoid the latency problems that wouldn't be a problem if you kept it on the same machine like SQL originally intended.