How to simulate multiple erlecopters in Gazebo?

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Hi everyone. Is there any particular way to, correctly, launch multiple erlecopters in a Gazebo simulation. I'm asking this because I tried to launch the .xacro file within a group like <group ns="ns1"> ... </group>

and even though everything appears to be well without errors, when I do rostopic list there several topics outside the ns1 namespace, some of which even appear to be duplicated. I tried to look in the .xacro files and the topics appear to be local, so they should come in the specified namespace. Please check below my launch file:

<?xml version='1.0'?>
<launch>
    <!-- start world -->

  <arg name="simRate" default="nan"/>
  <!-- Enable simulation clock -->
  <param name="use_sim_time" type="bool" value="true" />


  <arg name="world_name" default="$(find multi_uav)/worlds/empty.world" />
  <arg name="gui" value="true" />
  <arg name="headless" value="false" /> 

  <env name="GAZEBO_MODEL_PATH" value="$(find ardupilot_sitl_gazebo_plugin)/meshes/meshes_sensors"/>

<group ns="ns1">  
    <include file="$(find mavros)/launch/mavlink_bridge.launch"></include>

    <arg name="namespace" default="ns1"/>   
    <arg name="enable_logging" default="true"/>
        <arg name="enable_ground_truth" default="true"/>
    <arg name="log_file" default="erlecopter"/>
    <include file="$(find mavros)/launch/apm_sitl.launch"></include>
    <arg name="name" default="erlecopter1"/>
    <arg name="model" default="$(find ardupilot_sitl_gazebo_plugin)/urdf/erlecopter_base.xacro"/>
    <arg name="tf_prefix" default="$(arg namespace)"/>
    <arg name="debug" default="true"/>
    <arg name="verbose" default="true"/>

  <!-- Initial pose for the drone -->
    <arg name="x" default="0.0"/> <!-- [m], positive to the North -->
    <arg name="y" default="0.0"/> <!-- [m], negative to the East -->
    <arg name="z" default="0.08"/> <!-- [m], positive Up -->
    <arg name="roll" default="0"/> <!-- [rad] -->
    <arg name="pitch" default="0"/> <!-- [rad] -->
    <arg name="yaw" default="3.1415"/> <!-- [rad], negative clockwise -->

  <!-- send the robot XML to param server -->
    <param name="robot_description" command="
    $(find xacro)/xacro.py '$(arg model)'
    enable_logging:=$(arg enable_logging)
    enable_ground_truth:=$(arg enable_ground_truth)
    log_file:=$(arg log_file)"
  />
    <param name="tf_prefix" type="string" value="$(arg tf_prefix)" />


  <!-- push robot_description to factory and spawn robot in gazebo -->
    <node name="spawn_erlecopter" pkg="gazebo_ros" type="spawn_model"
   args="-param robot_description
         -urdf
         -x $(arg x)
         -y $(arg y)
         -z $(arg z)
         -R $(arg roll)
         -P $(arg pitch)
         -Y $(arg yaw)
         -model $(arg name)"
   respawn="false" output="screen" >
    </node> 
</group>
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="paused" value="true"/>   <!-- value unrelevant due to Arducopter plugin steps --> 
    <arg name="headless" value="$(arg headless)"/>
    <arg name="gui" value="$(arg gui)"/>
    <arg name="world_name" value="$(arg world_name)"/>
  </include>

</launch>

I based my launch file on the erlecopter_spawn.launch and the topics I get are

/clock
/command/motors
/command/rate
/diagnostics
/erlecopter/command/attitude
/erlecopter/command/motor_speed
/erlecopter/fix
/erlecopter/fix_velocity
/erlecopter/gazebo/command/motor_speed
/erlecopter/ground_truth/imu
/erlecopter/ground_truth/odometry
/erlecopter/ground_truth/pose
/erlecopter/ground_truth/pose_with_covariance
/erlecopter/ground_truth/position
/erlecopter/ground_truth/transform
/erlecopter/imu
/erlecopter/motor_speed/0
/erlecopter/motor_speed/1
/erlecopter/motor_speed/2
/erlecopter/motor_speed/3
/erlecopter/motors
/erlecopter/waypoint
/erlecopter/wind
/gazebo/link_states
/gazebo/model_states
/gazebo/parameter_descriptions
/gazebo/parameter_updates
/gazebo/set_link_state
/gazebo/set_model_state
/imu
/ns1/erlecopter/bottom/camera_info
/ns1/erlecopter/bottom/image_raw
/ns1/erlecopter/bottom/image_raw/compressed
/ns1/erlecopter/bottom/image_raw/compressed/parameter_descriptions
/ns1/erlecopter/bottom/image_raw/compressed ...