TYPO3 Core

A fixed method could look like this.

    static protected function removeDuplicatesForInsertion($insertionList, $list = '') {
        $insertionListParts = preg_split('/\\s*,\\s*/', $insertionList);
        $cleanInsertionListParts = array();
        foreach ($insertionListParts as $fieldName) {
            if ($fieldName == '--linebreak--' || !in_array($fieldName, $cleanInsertionListParts)) {
                $cleanInsertionListParts[] = $fieldName;
            }
        }
        $cleanInsertionList = implode(', ', $cleanInsertionListParts);
        if ($list === '') {
            return $cleanInsertionList;
        }
        // Get a list of fieldNames that are present in the list.
        preg_match_all('/(?:^|,)\\s*\\b([^;,]+)\\b[^,]*/', $list, $listMatches);
        // Remove the field names from the insertionlist.
        $fieldReplacePatterns = array();
        foreach ($listMatches[1] as $fieldName) {
            if ($fieldName != '--linebreak--') {
                $fieldReplacePatterns[] = '/(?:^|,)\\s*\\b' . preg_quote($fieldName, '/') . '\\b[^,$]*/';
            }
        }
        return preg_replace($fieldReplacePatterns, '', $cleanInsertionList);
    }

Anyway I think this method is incomplete because it does not check for duplicate fields respecting all palettes and normal fields. So imho its call could be removed from addFieldsToPalette() and executePositionedStringInsertion() what would fix the problem too.

Sorry, swaped direction in patch-1.patch.
Please use patch-2.patch!

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/34019