CollectionView return the same images twice - swift

Question

When I request the images and load them to my collection View, the request return low resolution images and high resolution images at the same time.
As you can see the same images are returned twice with low and hight resolution in my collectionView.
Code:
var imageArray = [UIImage]()
let imgManager = PHImageManager.default()
override func viewDidLoad() {
super.viewDidLoad()
collectionView.delegate = self
collectionView.dataSource = self
grabPhoto()
}
func grabPhoto(){
let imageSize = CGSize(width: 800, height: 800)
let fetchOptions = PHFetchOptions()
fetchOptions.sortDescriptors = [NSSortDescriptor(key: "creationDate", ascending: false)]
if let fetchResult = PHAsset.fetchAssets(with: .image, options: fetchOptions) as? PHFetchResult {
if fetchResult.count > 0 {
for i in 0..<fetchResult.count {
imgManager.requestImage(for: fetchResult.object(at: i), targetSize: imageSize, contentMode: .aspectFill , options: nil, resultHandler: { (image, error) in
self.imageArray.append(image!)
self.photoImageView.image = self.imageArray.first
self.collectionView.reloadData()
print("Result Size Is \(image?.size)")
})
}
}
} else {
print("you got no Photos!")
}
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return imageArray.count
}
func numberOfSections(in collectionView: UICollectionView) -> Int {
return 1
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "UploadPhotoCell", for: indexPath)
let imageView = cell.viewWithTag(1) as! UIImageView
imageView.image = imageArray[indexPath.row]
return cell
}