Fix popover not displayed reliably when VPN shortcut is unpinned (#2606)

Task/Issue URL: https://app.asana.com/0/1203137811378537/1207059171151302/f
Tech Design URL:
CC:

Description:

When the VPN shortcut is unpinned & the user tries to open the popover from the More Options menu, the temporarily shown VPN shortcut is hidden prematurely and (?) cause the popover to be closed right away.

DuckDuckGo/NavigationBar/View/NavigationBarViewController.swift

@@ -973,6 +973,7 @@ extension NavigationBarViewController: NSMenuDelegate {
             .store(in: &cancellables)
 
         networkProtectionButtonModel.$showButton
+            .removeDuplicates()
             .receive(on: RunLoop.main)
             .sink { [weak self] show in
                 let isPopUpWindow = self?.view.window?.isPopUpWindow ?? false