gui: Fix duplicate wallet showing up

The slot BitcoinGUI::addWallet can be invoked twice for the same
WalletModel due to a concurrent wallet being loaded after the first `connect()`:

```cpp
 connect(wallet_controller, &WalletController::walletAdded, this, &BitcoinGUI::addWallet);
 connect(wallet_controller, &WalletController::walletRemoved, this, &BitcoinGUI::removeWallet);

 for (WalletModel* wallet_model : m_wallet_controller->getOpenWallets()) {
     addWallet(wallet_model);
```

src/qt/bitcoingui.cpp

@@ -632,10 +632,10 @@ void BitcoinGUI::setWalletController(WalletController* wallet_controller)
 void BitcoinGUI::addWallet(WalletModel* walletModel)
 {
     if (!walletFrame) return;
+    if (!walletFrame->addWallet(walletModel)) return;
     const QString display_name = walletModel->getDisplayName();
     setWalletActionsEnabled(true);
     rpcConsole->addWallet(walletModel);
-    walletFrame->addWallet(walletModel);
     m_wallet_selector->addItem(display_name, QVariant::fromValue(walletModel));
     if (m_wallet_selector->count() == 2) {
         m_wallet_selector_label_action->setVisible(true);

src/qt/walletframe.cpp

@@ -39,11 +39,11 @@ void WalletFrame::setClientModel(ClientModel *_clientModel)
     this->clientModel = _clientModel;
 }
 
-void WalletFrame::addWallet(WalletModel *walletModel)
+bool WalletFrame::addWallet(WalletModel *walletModel)
 {
-    if (!gui || !clientModel || !walletModel) return;
+    if (!gui || !clientModel || !walletModel) return false;
 
-    if (mapWalletViews.count(walletModel) > 0) return;
+    if (mapWalletViews.count(walletModel) > 0) return false;
 
     WalletView *walletView = new WalletView(platformStyle, this);
     walletView->setBitcoinGUI(gui);
@@ -67,6 +67,8 @@ void WalletFrame::addWallet(WalletModel *walletModel)
     });
 
     connect(walletView, &WalletView::outOfSyncWarningClicked, this, &WalletFrame::outOfSyncWarningClicked);
+
+    return true;
 }
 
 void WalletFrame::setCurrentWallet(WalletModel* wallet_model)

src/qt/walletframe.h

@@ -36,7 +36,7 @@ public:
 
     void setClientModel(ClientModel *clientModel);
 
-    void addWallet(WalletModel *walletModel);
+    bool addWallet(WalletModel *walletModel);
     void setCurrentWallet(WalletModel* wallet_model);
     void removeWallet(WalletModel* wallet_model);
     void removeAllWallets();