The 'HTTP Duplicated Header' detection rule is designed to identify HTTP requests that contain multiple instances of the same header field, which can indicate possible HTTP request smuggling attacks. This anomaly-based rule targets the manipulation of header fields like 'Content-Length' and 'Transfer-Encoding' employed by attackers to manipulate the parsing of HTTP requests at different layers of web application infrastructures. According to RFC7230, the specification governing HTTP/1.1, senders must avoid generating multiple header fields with the same name unless the entire value is defined as a comma-separated list or it's a well-known exception. The rule parses request headers from Suricata logs, looking for headers that appear more than once while ignoring specific headers like 'Set-Cookie'. It aggregates such occurrences and flags them for further analysis. Proper implementation relies on ensuring that relevant web data models are adequately populated through supported technology add-ons for Splunk. False positives are not anticipated but may require tuning based on the organization’s logs. Key references include educational resources about request smuggling risks and methodologies. This rule plays a crucial role in enhancing web security posture against sophisticated attack vectors that exploit HTTP protocol ambiguities.