What if a GET request with _primary_shard preference land on a node which only have replica shards?

I was getting duplicate records in ES 2.2.1. So, I started using preference _primary_first for GET requests. But, I notice that there is no request landing on my 4th node in a cluster of 4 nodes. This node consist of two replicas.

Is it the case that node with replica will never cater to the requests coming with _primary_first preference?

Reference

The operation will be executed on primary shards if possible, but will fall back to other shards if not

This is the behavior of _primary_first. Reason for no requests on replica-only-node is that all of your requests are resolved by primary modes and no need was there to be executed on replica-only node.

And you are using very older version.

Regarding your duplicate documents:

Refer, Refer

You should mainly look at routing to identify the cause for the duplicates.