[css-properties-values-api-1] Multiple custom `@property` with the same name

[cdoublev]

This suggests that they are equivalent to running multiple registerProperty() with the same name:

Valid @property rules result in a registered custom property, as if registerProperty() had been called with equivalent parameters.

https://drafts.css-houdini.org/css-properties-values-api/#at-property-rule

I suggest that you make it clear that @property can override a definition, which I understood from reading other issues, whereas registerProperty() can not:

If property set already contains an entry with name as its property name (compared codepoint-wise), throw an InvalidModificationError and exit this algorithm.