Fix(Identity): Prevent duplicate welcome emails during account activation

(cherry picked from commit 1ba2399)

Author: malex1306

src/Smartstore.Web/Controllers/IdentityController.cs

@@ -361,6 +361,12 @@ public async Task<IActionResult> AccountActivation(string token, string email)
                 return RedirectToRoute("Homepage");
             }
 
+            if (await _userManager.IsEmailConfirmedAsync(customer))
+            {
+                ViewBag.ActivationResult = T("Account.AccountActivation.Activated");
+                return View();
+            }
+
             // Validate token & set user to active.
             var confirmed = await _userManager.ConfirmEmailAsync(customer, token);