A duvet should be warm without being heavy. When it weighs too much, it can feel oppressive, not from warmth, but from pressure. These are two different things.
One duvet is enough
For those who want to sleep light and comfortable, one duvet is enough all year round.
Two directions
An airy down construction works in two directions: it holds warmth when it is cold, and releases excess warmth when it grows warmer. A stable sleep climate, without switching.
The exception
Whoever grows warm very quickly chooses a separate summer duvet with less filling. That is the exception, not the rule.
The message
For most, the message is simple: one light duvet is enough through the year. Simplicity has its own elegance.