[css-minifier]custom properties(CSS variables) deduplication

[yisibl]

Input

.foo {
    --some-css-variable: red;
}

.foo {
    --some-css-variable: blue;
}

👇🏻 For display purposes, I have retained line breaks and spaces.

Expected output

.foo {
    --some-css-variable: blue;
}

Actual output

.foo {
    --some-css-variable: red;
}

.foo {
    --some-css-variable: blue;
}

See also: parcel-bundler/lightningcss#466