Header X-Robots-Tag being set twice in Express.js

I am trying to add X-Robots-Tag: all as a header to all requests on my express app. I have this middle ware code;

this.app.use(function(req, res, next) {
    res.setHeader('x-robots-tag', 'all')
    next();
});

I have also tried removing the x-robots-tag: none header then adding x-robots-tag: all it does not change the result.