docs(css): duplicate @import behavior (#16304)

Author: sapphi-red

docs/config/shared-options.md

@@ -292,6 +292,10 @@ Whether to enable sourcemaps during dev.
 
 Selects the engine used for CSS processing. Check out [Lightning CSS](../guide/features.md#lightning-css) for more information.
 
+::: info Duplicate `@import`s
+Note that postcss (postcss-import) has a different behavior with duplicated `@import` from browsers. See [postcss/postcss-import#462](https://github.com/postcss/postcss-import/issues/462).
+:::
+
 ## css.lightningcss
 
 - **Experimental:** [Give Feedback](https://github.com/vitejs/vite/discussions/13835)