From https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2:

Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.

A HashMap<String, String> is currently used for response headers which will fail to handle this case correctly. That is,

Header-Name: Value1
Header-Name: Value2

will currently be collapsed into

Header-Name: Value 2

when sent from ProtocolServer, but it should either be passed with the duplicate fields intact, or collapsed into

Header-Name: Value1, Value2

before being sent across the IPC (if Header-Name is defined as a comma-separated list). I'm not sure what should happen for duplicate headers that are not comma-separated lists.