SFSpeechRecognizer returns same result multiple times.

In our demo app we tried to use SFSpeechRecognizer to get functionality speech-to-text, however it always returns same value in

result?.bestTranscription.formattedString multiple times also sometimes it won't recognize the word correctly. Can you please help me?

Existing Code (Swift 4.2):

func startRecording(_ button:UIButton) {
        if recognitionTask != nil {
            recognitionTask?.cancel()
            recognitionTask = nil
        }

        let audioSession = AVAudioSession()
        do {
            try audioSession.setCategory(.record, mode: .measurement, options: [])
            try audioSession.setActive(true, options: .notifyOthersOnDeactivation)
        } catch {
            print("audioSession properties weren't set because of an error.")
        }

        recognitionRequest = SFSpeechAudioBufferRecognitionRequest()
        let inputNode = audioEngine.inputNode

        guard let recognitionRequest = recognitionRequest else {
            fatalError("Unable to create an SFSpeechAudioBufferRecognitionRequest object")
        }

        recognitionTask = speechRecognizer.recognitionTask(with: recognitionRequest, resultHandler: { [unowned self] (result, error) in
            
            var isFinal = false

            if result != nil {
                self.value = (result?.bestTranscription.formattedString)!
                print("Value received is : \(self.value)")
                isFinal = (result?.isFinal)!
            }

            if error != nil || isFinal {
                inputNode.removeTap(onBus: 0)
                self.recognitionRequest = nil
                self.recognitionTask = nil
                self.recognitionTask?.cancel()
                button.isEnabled = true
            }
        })

        let recordingFormat = inputNode.outputFormat(forBus: 0)
        inputNode.installTap(onBus: 0, bufferSize: 1024, format: recordingFormat) { (buffer, when) in
            self.recognitionRequest?.append(buffer)
        }

        audioEngine.prepare()

        do {
            try audioEngine.start()
        } catch {
            print("audioEngine couldn't start because of an error.")
        }
    }