How to repeat equal data using data.map

I'm using the d3.js library and wanted it to read equal data, not overwrite the scales.

Here is the code snippet that does this, and an example of my .json file:

const months = data.map(function(d) {
            //console.log(d.month);
            return d.month;
        });
        //console.log(months);

In case the constant months is receiving an array.

const xScale = d3.scaleBand()
            .rangeRound([0, w])
            .domain(months);

But when I use the function d3.ScaleBand() it overwrites data that is the same, but I wish it did not.

svg.append('g')
    .attr('transform', 'translate(' + marginLeft + ',' + marginRight +')')
    .call(d3.axisLeft(yScale));

.json file:

[
  {"month": "January", "days": 30},
  {"month": "February", "days": 28},
  {"month": "March", "days": 31},
  {"month": "April", "days": 30},
  {"month": "May", "days": 15},
  {"month": "June", "days": 30},
  {"month": "July", "days": 22},
  {"month": "August", "days": 11},
  {"month": "September", "days": 30},
  {"month": "October", "days": 8},
  {"month": "November", "days": 30},
  {"month": "December", "days": 25},
  {"month": "September", "days": 3},
  {"month": "October", "days": 18}
]

In this case I repeated the months "September" and "October" at the end, but when drawing the scales, it draws on the existing ones.