rpc: return "already mining" in start_mining if already mining

src/rpc/core_rpc_server.cpp

@@ -863,7 +863,13 @@ namespace cryptonote
     boost::thread::attributes attrs;
     attrs.set_stack_size(THREAD_STACK_SIZE);
 
-    if(!m_core.get_miner().start(info.address, static_cast<size_t>(req.threads_count), attrs, req.do_background_mining, req.ignore_battery))
+    cryptonote::miner &miner= m_core.get_miner();
+    if (miner.is_mining())
+    {
+      res.status = "Already mining";
+      return true;
+    }
+    if(!miner.start(info.address, static_cast<size_t>(req.threads_count), attrs, req.do_background_mining, req.ignore_battery))
     {
       res.status = "Failed, mining not started";
       LOG_PRINT_L0(res.status);