Merge pull request #135 from paritytech/lexnv/unpin-duplicate-hashes

chainHead/unpin: Return error on duplicate hashes

src/api/chainHead_unstable_unpin.md

@@ -3,7 +3,7 @@
 **Parameters**:
 
 - `followSubscription`: An opaque string that was returned by `chainHead_unstable_follow`.
-- `hashOrHashes`: String or array of strings containing the hexadecimal-encoded hash of the header of the block to unpin.
+- `hashOrHashes`: String or array of unique strings containing the hexadecimal-encoded hash of the header of the block to unpin.
 
 **Return value**: *null*
 
@@ -18,5 +18,6 @@ If this function returns an error, then no block has been unpinned. An JSON-RPC
 ## Possible errors
 
 - A JSON-RPC error with error code `-32801` is generated if the `followSubscription` is valid but at least one of the block hashes passed as parameter doesn't correspond to any block that has been reported by `chainHead_unstable_follow`, or at least one of the block hashes has been unpinned.
+- A JSON-RPC error with error code `-32804` is generated if the `hashOrHashes` parameter is an array and at least one of the block hashes is duplicated.
 - A JSON-RPC error with error code `-32602` is generated if one of the parameters doesn't correspond to the expected type (similarly to a missing parameter or an invalid parameter type).
 - No error is generated if the `followSubscription` is invalid or stale. The call is simply ignored.