Duplicate Child Node Handler

[adrianmross]

https://github.com/adrianmross/standalone-scp/blob/464219e350e8be047590915242ce1099d86ace6e/parser.cpp#L139-L145

Currently, there is no handler on duplicate children nodes. It should throw an error.

[zek224]

Added a unique node string-set in parser.cpp. If the node is found in the set, it doesn't get added to the quorum vector

        // set to track unique nodes in the quorum slice
        set<string> unique_nodes;
        unique_nodes.insert(node_to_name[i]);           // Add the current node to the set

        for(const string& node_name : quorum_names) {
            // check if the node is already in the set
            if(unique_nodes.find(node_name) == unique_nodes.end()) {
                stellar::NodeID curr_node = name_to_node[node_name];
                quorum_vec.push_back(curr_node);
                unique_nodes.insert(node_name); // Add the node to the set
            }
        }
        node_to_quorum[i] = initQSet(quorum_vec);