fix(compiler-sfc): duplicated injected css var with repeated vars

[patak-dev]

When the same css var was used multiple times in <style>, that var was injected several times in the <script>

For the added test case it was generating

_useCssVars(_ctx => ({
  "xxxxxxxx-color": (color),
  "xxxxxxxx-color": (color)
}))

This PR removes duplicated vars in parseCssVars so we get

_useCssVars(_ctx => ({
  "xxxxxxxx-color": (color)
}))