This is the main reason I love Gel: https://www.geldata.com/

It's a "new database" (actually a layer on top of Postgres). It has a more ergonomic query language, and it returns results as nested structures rather than rows. So "give me all my customers with a list of their orders" returns what you want.

Unfortunately, the startup that was developing Gel got acqui-hired, and Gel's future is unclear.

If anything a well designed database minimises data duplication. If your query returns duplicate data that's on the writer, not the language's fault.