Throw error for no-op egress enablement

Signed-off-by: nshankar <nshankar@microsoft.com>

Author: nshankar13

src/aks-preview/azext_aks_preview/managed_cluster_decorator.py

@@ -2402,6 +2402,12 @@ def _handle_egress_gateways_asm(self, new_profile: ServiceMeshProfile) -> Tuple[
                             f'Egress gateway {istio_egressgateway_name} '
                             f'in namespace {istio_egressgateway_namespace} is already disabled.'
                         )
+                    if egress.enabled and enable_egress_gateway and egress.gateway_configuration_name == gateway_configuration_name:
+                        raise ArgumentUsageError(
+                            f'Egress gateway {istio_egressgateway_name} '
+                            f'in namespace {istio_egressgateway_namespace} is already enabled '
+                            f'with gateway configuration name {gateway_configuration_name}.'
+                        )
                     egress.enabled = enable_egress_gateway
                     # only update gateway configuration name for enabled egress gateways
                     if enable_egress_gateway: