wkwebViewでProgress barが２つ表示されてしまう

以下のコードでwebviewの初回読み込み時には、progress barが意図した動きになるのですが、webview内のリンクをふむとprogress barが２つ表示されているように見えてしまいます。
どなたかおわかりになる方はいらっしゃいませんでしょうか？

Swift
1class DetailViewController: UIViewController {
2    
3    private var wkWebView: WKWebView!
4    
5    let progressView = UIProgressView(progressViewStyle: .default)
6    
7    private var estimatedProgressObserver: NSKeyValueObservation?
8    
9    override func viewDidLoad() {
10        super.viewDidLoad()
11
12        setupWebView()
13        load()
14        setupProgressView()
15        setupEstimatedProgressObserver()
16    }
17 
18
19
20    func setupWebView() {
21        let webConfig = WKWebViewConfiguration()
22        wkWebView = WKWebView(frame: .zero, configuration: webConfig)
23        wkWebView.uiDelegate = self
24        wkWebView.navigationDelegate = self
25    }
26
27
28    private func setupProgressView() {
29        guard let navigationBar = navigationController?.navigationBar else { return }
30
31        progressView.translatesAutoresizingMaskIntoConstraints = false
32        navigationBar.addSubview(progressView)
33
34        progressView.isHidden = true
35
36        NSLayoutConstraint.activate([
37            progressView.leadingAnchor.constraint(equalTo: navigationBar.leadingAnchor),
38            progressView.trailingAnchor.constraint(equalTo: navigationBar.trailingAnchor),
39
40            progressView.bottomAnchor.constraint(equalTo: navigationBar.bottomAnchor),
41            progressView.heightAnchor.constraint(equalToConstant: 2.0)
42        ])
43    }
44    
45    private func setupEstimatedProgressObserver() {
46        estimatedProgressObserver = wkWebView.observe(.estimatedProgress, options: [.new]) { [weak self] webView, _ in
47            self?.progressView.progress = Float(webView.estimatedProgress)
48        }
49    }
50}
51
52
53// MARK: - WKNavigationDelegate
54extension DetailViewController: WKNavigationDelegate {
55    
56  
57    // 読み込み準備開始
58    func webView(_ webView: WKWebView, didStartProvisionalNavigation navigation: WKNavigation!) {
59        if progressView.isHidden {
60            progressView.isHidden = false
61        }
62
63        UIView.animate(withDuration: 0.33, animations: {
64            self.progressView.alpha = 1.0
65        })
66        print("読み込み準備開始")
67    }
68
69    // 読み込み完了
70    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
71        UIView.animate(withDuration: 0.33,
72                       animations: {
73                           self.progressView.alpha = 0.0
74                       },
75                       completion: { isFinished in
76                           self.progressView.isHidden = isFinished
77        })
78        print("読み込み完了")
79    }
80}