Set Id property on existing devices so we don't try to create a new one instead of updating existing. (#2420)

src/Identity/IdentityServer/BaseRequestValidator.cs

@@ -562,6 +562,10 @@ public abstract class BaseRequestValidator<T> where T : class
                     }
                 }
             }
+            else
+            {
+                deviceFromRequest.Id = existingDevice.Id;
+            }
 
             deviceFromRequest.UserId = user.Id;
             await _deviceService.SaveAsync(deviceFromRequest);