e.g.:
function Render() {
// First sort the stylesheets by key
if (is_array($this->StyleSheets)) {
ksort($this->StyleSheets);

// Remove duplicate values
// Can't use directly array_unique (only works on strings)
foreach ($this->StyleSheets as $Key => $Value) {
$this->StyleSheets[$Key] = serialize($Value);
}
$this->StyleSheets = array_unique($this->StyleSheets);
foreach ($this->StyleSheets as $Key => $Value) {
$this->StyleSheets[$Key] = unserialize($Value);
}
}
if (is_array($this->Scripts)) {
// Remove duplicate values
$this->Scripts = array_unique($this->Scripts);
}
$this->CallDelegate('PreRender');
include(ThemeFilePath($this->Context->Configuration, 'head.php'));
$this->CallDelegate('PostRender');
}

Array unique only work with string value.
To remove them in $this->StyleSheets, we would have serialize all the value first.

It does. It's added for script.

I didn't change the AddStyleSheet(). Duplicate style sheets won't happen that often (that could since there are css frameworks out there and js framework can have style sheets) and since they cascade not adding a style sheet could change the style of a page, I don't know what it should do (not add? or remove the previous one?).