check_manifest_dependencies: dependency added when already present

[Achllle]

I'm not sure if this is different from #24, adding separate issue for it in case it is

Starting with the following package.xml:

<?xml version="1.0"?>
<package format="2">
  <name>mypkg</name>
  <version>0.0.0</version>
  <description>description</description>
  <maintainer email="person@org.com">person</maintainer>

  <license>TODO</license>

  <depend>rospy</depend>
  <depend>std_msgs</depend>

  <buildtool_depend>catkin</buildtool_depend>

  <build_depend>message_generation</build_depend>
  <build_export_depend>message_runtime</build_export_depend>
  <exec_depend>message_runtime</exec_depend>

</package>

Running glint_ros -f mypkg check_manifest_dependencies suggests:

+   <depend>rospy</depend>
    <depend>rospy</depend>

which of course prevents building:

- The generic dependency on 'rospy' is redundant with: build_depend, build_export_depend, exec_depend

EDIT: I should note that this happened after rerunning the tool after fixing the spurious build_export_depend flag from #24.