Proper way to add and remove `SKPaymentQueue` observer Swift

Question

I am adding the observer in the didFinishLaunchingWithOptions like this and then call receipt validation:

IAPManager.shared.startObserving()
IAPManager.shared.IAPResponseCheck(iapReceiptValidationFrom: .didFinishLaunchingWithOptions)

And removing it in the applicationWillTerminate like this:

IAPManager.shared.stopObserving()

I am also checking the state of the purchase at applicationWillEnterForeground by calling the receipt validation:

IAPManager.shared.IAPResponseCheck(iapReceiptValidationFrom: .applicationWillEnterForeground)

IAP Manager class in short:

class IAPManager: NSObject {
    static let shared = IAPManager()     
    private override init() { super.init() }

    func startObserving() {
        SKPaymentQueue.default().add(self)
    }

    func stopObserving() {
        SKPaymentQueue.default().remove(self)
    }
}

In the IAP manager class inside of updatedTransactions, I am verifying the receipt and then finishing the transaction after each purchase & restore like this:

case .purchased:
    self.IAPResponseCheck(iapReceiptValidationFrom: .purchaseButton)
    SKPaymentQueue.default().finishTransaction(transaction)
    
case .restored:
    totalRestoredPurchases += 1
    SKPaymentQueue.default().finishTransaction(transaction)

And lastly call the receipt validation inside of paymentQueueRestoreCompletedTransactionsFinished:

func paymentQueueRestoreCompletedTransactionsFinished(_ queue: SKPaymentQueue) {
    if totalRestoredPurchases != 0 {
        self.IAPResponseCheck(iapReceiptValidationFrom: .restoreButton)
    } else {
        print("IAP: No purchases to restore!")
        onBuyProductHandler?(.success(false))
    }
}

But my receipt validation Func gets called multiple times randomly when I came back from background to foreground, or restart the app.

func IAPResponseCheck(iapReceiptValidationFrom: IAPReceiptValidationFrom) {
        print("iapReceiptValidationFrom \(iapReceiptValidationFrom)")

}

I search on the internet I found that it is happening because somehow there are multiple observers are added. But I add it according to apple's guidelines. So according to my implementation what am I missing here?

I want to call my receipt validation func just one time