Du You's method was not really kept secret. Creating doppelgangers is something anyone above the seventh tier can do; it's just a matter of proficiency.
After learning that Du You used a doppelganger to overthrow a ninth-tier world, everyone began to study it.
After all, it's just a doppelganger; even if it gets destroyed, it won't significantly impact oneself. But if successful, the results could be incredibly significant.
Even though not everyone excels at the Soul Law, nor does everyone have Du You's powerful deductive abilities, they don't all have to target ninth-tier worlds; it's worth trying.
During the comprehensive discussion about entering this new world, many people above the seventh tier started to mimic.
And the price of various doppelganger creation secret methods began to rise, along with the cost of suitable materials. This impact isn't limited to those above the seventh tier; even ordinary markets felt it.
