Remove duplicate numbers from an array without using javascript methods ?

const numbers = [1,1,2,3,3,4,5,5,6,7,7,7,8,8,9];

const duplicateNumber =[];

function removeDuplicateNumber(arr){
for(let i=0; i< arr.length; i++){
if(!duplicateNumber.includes(arr[i])){
duplicateNumber.push(arr[i]);
}
}
console.log(duplicateNumber);
}

removeDuplicateNumber(numbers);