error while launching robot model

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Hi all, I am trying to launch a robot model in gazebo(using ros indigo and ubuntu 14.04). But a node 'spawn_model' which is specified in launch file is automatically killed while running roslaunch and robot model is not yet launched in gazebo . My urdf file is fine and I checked it using gzsdf tool and by launching the model in Rviz. So I edited the launch file by adding respawn="true". So I am getting error "[ERROR] [1506783269.127628154, 11.257000000]: SpawnModel: Failure - model name hobo already exist.". What does this error mean? please help

C:\fakepath\Screenshot from 2017-09-30 20:28:28.png

launch file is :

<launch>
<param name="robot_description" textfile="$(find hobo_description)/urdf/hobo_description.urdf" />
<include file="$(find gazebo_ros)/launch/empty_world.launch">
</include>
<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" respawn="true" args="-param robot_description -urdf -model hobo" />

</launch>

urdf file is :

<?xml version="1.0"?>
<robot name="hobo">
<link name="base_link">
<visual>
    <geometry>
        <cylinder length="0.01" radius="0.25"/>
    </geometry>
    <material name="silver">
        <color rgba="0.75 0.75 0.75 1"/>
    </material>
</visual>
<collision>
    <geometry>
        <cylinder length="0.01" radius="0.25"/>
    </geometry>
</collision>
</link>

<link name="left_wheel">
<visual>
    <geometry>
        <cylinder length="0.05" radius="0.05"/>
    </geometry>
    <material name="black"/>            
</visual>
<collision>
    <geometry>
        <cylinder length="0.05" radius="0.05"/>
    </geometry>
</collision>
</link>

<joint name="left_wheel_to_base" type="continuous">
<axis xyz="0 0 1"/>
<parent link="base_link"/>
<child link="left_wheel"/>
<origin rpy="0 -1.5708 0" xyz="-0.275 0 0"/>
<inertial>
<mass value="1.0"/>
<inertia ixx="5.1458e-5" iyy="5.1458e-5" izz="5.1458e-5" ixy="0" ixz="0" iyz="0"/>
</inertial>
</joint>

<link name="right_wheel">
<visual>
    <geometry>
        <cylinder length="0.05" radius="0.05"/>
    </geometry>
    <material name="black"/>            
</visual>
<collision>
    <geometry>
        <cylinder length="0.05" radius="0.05"/>
    </geometry>
</collision>
</link>

<joint name="right_wheel_to_base" type="continuous">
<axis xyz="0 0 1"/>
<parent link="base_link"/>
<child link="right_wheel"/>
<origin rpy="0 -1.5708 0" xyz="0.275 0 0"/>
<inertial>
<mass value="0.1"/>
<inertia ixx="5.1458e-5" iyy="5.1458e-5" izz="5.1458e-5" ixy="0" ixz="0" iyz="0"/>
</inertial>
</joint>


<link name="box">
<visual>
    <geometry>
        <box size="0.2 0.2 0.2"/>
    </geometry>
    <material name="orange"/>           
</visual>
<collision>
    <geometry>
        <box size="0.2 0.2 0.2"/>
    </geometry>
</collision>
</link>

<joint name="box_to_base" type="fixed">
<parent link="base_link"/>
<child link="box"/>
<origin xyz="0 0 0.1"/>
<inertial>
<mass value="1.0"/>
<inertia ixx="0.015" iyy="0.0375" izz="0.0375" ixy="0" ixz="0" iyz="0"/>
</inertial>
</joint>
</robot>