Don't add a new image layer with remote control if it already loaded

[pyrog]

What steps will reproduce the problem?

1. start josm and load the Bing imagery
2. load Bing imagery with remote ​http://127.0.0.1:8111/imagery?type=bing&url=http://www.bing.com/maps/

What is the expected result?

The Bing imagery should be loaded once in JOSM

What happens instead?

The Bing imagery is loaded twice in JOSM

Please provide any additional information below. Attach a screenshot if possible.

This issue append with any imagery layer.
This more a enhancement that a real issue ;-)

JOSM is remotely controlled by ​HOT Task manager to edit map in Humanitarian projects.

See also issue #10798

Revision: 7777
Repository Root: http://josm.openstreetmap.de/svn
Relative URL: ^/trunk
Last Changed Author: Don-vip
Last Changed Date: 2014-12-09 18:14:49 +0100 (Tue, 09 Dec 2014)
Build-Date: 2014-12-09 17:17:36
URL: http://josm.openstreetmap.de/svn/trunk
Repository UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last Changed Rev: 7777

Identification: JOSM/1.5 (7777 en) Mac OS X 10.10.1
Memory Usage: 382 MB / 910 MB (255 MB allocated, but free)
Java version: 1.8.0_25, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM

Plugins:
- ImageWayPoint (30737)
- OpeningHoursEditor (30737)
- PicLayer (30762)
- cadastre-fr (30762)
- geotools (30762)
- gpxfilter (30738)
- imagery_offset_db (30808)
- jts (30762)
- measurement (30737)
- notes (v0.9.5)
- opendata (30806)
- reverter (30737)
- scripting (30702)
- tag2link (30719)
- undelete (30762)
- utilsplugin2 (30762)
- wikipedia (30780)

Last errors/warnings:
- E: Failed to locate image 'Icon_uzkb.png'
- W: Could not get presets icon Icon_uzkb.png
- E: java.net.ConnectException: Network is down
- W: org.openstreetmap.josm.io.OsmTransferException: Could not connect to the OSM server. Please check your internet connection.. Cause: java.net.ConnectException: Network is down
- W: Unknown remote control parameter source, skipping it

[pyrog]

See ​https://github.com/hotosm/osm-tasking-manager2/issues/450#issuecomment-67773334

[Don-vip]

In 7871/josm:

fix #10878 - do not add new imagery layer through remote control if it already exists