Although named CssSpecificity, it finds dupes. Aims to find over-specificityfied (and so unused) rules.

css-specificity.php

<?php
/**
 * Some Css-Tests
 */
class CssSpecificity {

    /**
     * Css-Content
     *
     * @var string
     */
    private $_sCss;

    /**
     * Found selectors
     *
     * @var array
     */
    private $_aSelectors;

    /**
     * Found dupes
     *
     * @var array
     */
    private $_aDupes = array(
        'selector' => array(),
        'rule' => array(),
    );

    /**
     * Load a css-file
     *
     * @param  string $sFile
     *
     * @return CssSpecificity
     */
    public function load($sFile) {
        if (file_exists($sFile)) {
            $this->_sCss .= file_get_contents($sFile);
        }

        return $this;
    }

    /**
     * Run
     *
     * @return CssSpecificity
     */
    public function run() {
        $this->_cleanup()->_parse()->_dupes();

        // print_r($this->_aSelectors);
        print_r($this->_aDupes);

        return $this;
    }

    /**
     * Cleanup the css
     *
     * @return CssSpecificity
     */
    private function _cleanup() {
        $v = str_replace("\r\n", "\n", trim($this->_sCss));
        $search = array(
            "/\/\*[\d\D]*?\*\/|\t+/",
            "/\s+/",
            "/\}\s+/"
        );
        $replace = array(
            null,
            " ",
            "}\n"
        );
        $v = preg_replace($search, $replace, $v);
        $search = array(
            "/\\;\s/",
            "/\s+\{\\s+/",
            "/\\:\s+\\#/",
            "/,\s+/i",
            "/\\:\s+\\\'/i",
            "/\\:\s+([0-9]+|[A-F]+)/i"
        );
        $replace = array(
            ";",
            "{",
            ":#",
            ",",
            ":\'",
            ":$1"
        );
        $v = preg_replace($search, $replace, $v);
        $v = str_replace("\n", null, $v);
        $this->_sCss = str_replace('}', '}' . PHP_EOL, $v);

        return $this;
    }

    /**
     * Parse the css to selectors, specificity and rules
     *
     * @return CssSpecificity
     */
    private function _parse() {
        $aRules = explode(PHP_EOL, $this->_sCss);
        foreach ($aRules as $iLine => $sRule) {
            $aParts = explode('{', $sRule);
            $aSelectors = explode(',', $aParts[0]);
            foreach ($aSelectors as $sSelector) {
                $sSelector = trim($sSelector);
                if (strlen($sSelector) > 0) {
                    $aStyles = explode(';', str_replace('}', '', $aParts[1]));
                    sort($aStyles);
                    $this->_aSelectors[] = array(
                        'selector' => $sSelector,
                        'specificity' => $this->_specificityfy($sSelector),
                        'rule' => implode('; ', $aStyles),
                        'line' => $iLine + 1,
                    );
                }
            }
        }

        return $this;
    }

    /**
     * Determine the specificity of a selector
     *
     * @param  string $sSelector
     *
     * @return int
     */
    private function _specificityfy($sSelector) {
        $aParts = explode(' ', $sSelector);
        $iSpecificity = 0;
        foreach ($aParts as $sPart) {
            $sPart = trim($sPart);
            $iSpecificity += (substr_count($sPart, '#') * 100);
            $iSpecificity += (substr_count($sPart, '.') * 10);
            $iSpecificity += (substr_count($sPart, ':') * 10);
            if (in_array($sPart{0}, array(
                '#',
                ':',
                '.'
            )) === false) {
                $iSpecificity++;
            }
        }

        return $iSpecificity;
    }

    /**
     * Determine dupes
     *
     * @return CssSpecificity
     */
    private function _dupes() {
        $aStorage = array(
            'selector' => array(),
            'rule'  => array()
        );

        $aTypes = array_keys($aStorage);
        foreach ($this->_aSelectors as $aSelector) {
            foreach ($aTypes as $sType) {
                if (isset($aStorage[$sType][$aSelector[$sType]]) !== true or $aStorage[$sType][$aSelector[$sType]] === $aSelector['line']) {
                    $aStorage[$sType][$aSelector[$sType]] = $aSelector['line'];
                }
                else {
                    $this->_addDupe($aSelector[$sType], $aStorage[$sType][$aSelector[$sType]], $aSelector['line'], $sType);
                }
            }
        }

        return $this;
    }

    /**
     * Add a dupe
     *
     * @param  string $sString
     * @param  int $iLineA
     * @param  int $iLineB
     * @param  string $sWhat
     *
     * @return CssSpecificity
     */
    private function _addDupe($sString, $iLineA, $iLineB, $sWhat = 'selector') {
        $this->_aDupes[$sWhat][] = array(
            'content' => $sString,
            'first' => $iLineA,
            'dupe' => $iLineB,
        );

        return $this;
    }
}

$aArgs = getopt('f:');
if (isset($aArgs['f']) !== true) {
    die('File(s) are missing' . PHP_EOL);
}

$aFiles = explode(',', $aArgs['f']);
$o = new CssSpecificity();
foreach ($aFiles as $sFile) {
    $o->load($sFile);
}

$o->run();