MIME-551 — A file extension occurs twice in the file defining MIME types
In HTTP protocol, server indicates to client the MIME type of response data using Content-Type header. If the header is missing, proxy tries to guess the type from the suffix of the request URI. During initialization, the proxy reads a file which defines mapping from suffixes to MIME types. This message is not a fatal error, it only indicates that the same extension (suffix) exists more than once in the definition file. All occurrences except the last one are ignored.