Fix: https://github.com/AlexStocks/getty/issues/27

Signed-off-by: AlexStocks <alexstocks@foxmail.com>

protocol/dubbo/pool.go

@@ -360,6 +360,13 @@ func (p *gettyRPCClientPool) put(conn *gettyRPCClient) {
 		return
 	}
 
+	// check whether @conn has existed in p.conns or not.
+	for i := range p.conns {
+		if p.conns[i] == conn {
+			return
+		}
+	}
+
 	if len(p.conns) >= p.size {
 		// delete @conn from client pool
 		// p.remove(conn)