Remove z_sendmany restriction on duplicate addresses

This is from https://github.com/zcash/zcash/issues/2955

and will potentially allow HushList protocol to send
up to 512*54 = 27KB of memo data to a single address without
requiring a multipart protocol. This is possible because
all memos will share the same txid and can be uniquely identified
and ordered by their transaction output index.

src/wallet/rpcwallet.cpp

@@ -1,6 +1,6 @@
 // Copyright (c) 2010 Satoshi Nakamoto
 // Copyright (c) 2009-2014 The Bitcoin Core developers
-// Copyright (c) 2017      The Hush developers
+// Copyright (c) 2017-2018    The Hush developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -3376,9 +3376,6 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
     if (outputs.size()==0)
         throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, amounts array is empty.");
 
-    // Keep track of addresses to spot duplicates
-    set<std::string> setAddress;
-
     // Recipients
     std::vector<SendManyRecipient> taddrRecipients;
     std::vector<SendManyRecipient> zaddrRecipients;
@@ -3408,10 +3405,6 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
             }
         }
 
-        if (setAddress.count(address))
-            throw JSONRPCError(RPC_INVALID_PARAMETER, string("Invalid parameter, duplicated address: ")+address);
-        setAddress.insert(address);
-
         UniValue memoValue = find_value(o, "memo");
         string memo;
         if (!memoValue.isNull()) {