Behavior of GmailMessage.getHeader(..)

[Ajay Shinde]

Hi All, 

In Google Apps script project to build an Gmail Add On,
If duplicate headers are present in gmail message then which will always get returned upon executing message.getHeader("duplicate-header-name")?

const onGmailMessageOpen= (e)=>{

var accessToken = e.gmail.accessToken;

GmailApp.setCurrentMessageAccessToken(accessToken);

var messageId = e.gmail.messageId;

var message = GmailApp.getMessageById(messageId);

console.log(message.getHeader("duplicate-header-name"));

}

Behavior seen by me is that it returns last header value if duplicate headers are present. But will it be always? 
In documentation it is not specified.