Overriding auto-discovered services with service entries


I’ve been trying to override/add few parameters of/in auto-discovered services by creating service entries with same host but getting “pilot_duplicate_envoy_clusters” error.
Is it not the correct way of achieving the intended behavior ? If not, could you please explain why ?
Also suggest a way to achieve the same if any.

k8s cluster have a service “service86.class8.svc.cluster.local” with no named port.
I’m trying to override above service by creating service entry with same host and a named port with protocol “HTTP2” so that istio-proxy initiates http2 connection to this service.