chainHead: Error on duplicate unpin hashes (#3313)

This PR addresses an issue where calling chainHead_unpin with duplicate
hashes could lead to unintended side effects.

This backports:
https://github.com/paritytech/json-rpc-interface-spec/pull/135

While at it, have added a test to check that the global reference count
is decremented only once on unpin.

cc @paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Davide Galassi <davxy@datawok.net>

substrate/client/rpc-spec-v2/src/chain_head/chain_head.rs

@@ -424,6 +424,8 @@ where
 				// Block is not part of the subscription.
 				Err(ChainHeadRpcError::InvalidBlock)
 			},
+			Err(SubscriptionManagementError::DuplicateHashes) =>
+				Err(ChainHeadRpcError::InvalidDuplicateHashes),
 			Err(_) => Err(ChainHeadRpcError::InvalidBlock),
 		}
 	}