SpawnModel: Failure - model name already exist

edit

It seems like you use ros to start your robot in this case you need to specify the model_name of the robot in the model field of the service.

if you use the hector_quadrotor you can specify the name in your launch file($(arg name) in the example:)

<include file="$(find hector_quadrotor_gazebo)/launch/spawn_quadrotor.launch">
  <arg name="name" default="$(arg name)"/>
  <arg name="model" value="$(arg model)"/>
  <arg name="use_ground_truth_for_tf" value="true"/>
  <arg name="use_ground_truth_for_control" value="true"/>

  <arg name="use_pose_estimation" value="false"/>

   <arg name="tf_prefix" default="$(arg tf_prefix)"/>
   <arg name="x" default="$(arg x)"/>
   <arg name="y" default="$(arg y)"/>
   <arg name="z" default="$(arg z)"/>
</include>