src/ModelAdmin/SEOEditorAdmin.php
private function markDuplicates($list)
    {
        $duplicates = $this->findDuplicates($list, 'MetaTitle')->map('ID', 'ID')->toArray();
        $duplicateList = new ArrayList();

        foreach ($list as $item) {
            if (in_array($item->ID, $duplicates)) {
                $item->IsDuplicate = true;
                $duplicateList->push($item);
            }
        }

        $duplicates = $this->findDuplicates($list, 'MetaDescription')->map('ID', 'ID')->toArray();
        foreach ($list as $item) {
            if (in_array($item->ID, $duplicates)) {
                $item->IsDuplicate = true;
                if (!$list->byID($item->ID)) {
                    $duplicateList->push($item);
                }
            }
        }

        $duplicateList->merge($list);
        $duplicateList->removeDuplicates();
        return $duplicateList;
    }
private function markDuplicates($list)
    {
        $duplicates = $this->findDuplicates($list, 'MetaTitle')->map('ID', 'ID')->toArray();
        $duplicateList = new ArrayList();

        foreach ($list as $item) {
            if (in_array($item->ID, $duplicates)) {
                $item->IsDuplicate = true;
                $duplicateList->push($item);
            }
        }

        $duplicates = $this->findDuplicates($list, 'MetaDescription')->map('ID', 'ID')->toArray();
        foreach ($list as $item) {
            if (in_array($item->ID, $duplicates)) {
                $item->IsDuplicate = true;
                if (!$list->byID($item->ID)) {
                    $duplicateList->push($item);
                }
            }
        }

        $duplicateList->merge($list);
        $duplicateList->removeDuplicates();
        return $duplicateList;
    }
Mark duplicate attributes

@param SS_List $list
@return SS_List