With plugin 2.13.1 we now see two devices pointing to the same emulator instance which caused our test runner (spoon) to think there are two distinct devices and fail no matter what. Here is the console output:
===begin copy & paste ===
[android] Using Android SDK: /opt/android/android-sdk-linux
[android] Creating Android AVD: /scratch/jenkins/workspace/Android Integration (DEV)/label/android/node_index/1/.android/avd/hudson_en-US_240_WVGA_android-16_armeabi-v7a_1.avd
[android] /opt/android/android-sdk-linux/tools/android create avd -f -a -c 64M -s WVGA800 -n hudson_en-US_240_WVGA_android-16_armeabi-v7a_1 -t android-16 --abi armeabi-v7a
$ /opt/android/android-sdk-linux/platform-tools/adb start-server

• daemon not running. starting it now on port 5938 *
• daemon started successfully *
  $ /opt/android/android-sdk-linux/platform-tools/adb start-server
  [android] Starting Android emulator
  $ /opt/android/android-sdk-linux/tools/emulator64-arm -ports 5936,5937 -prop persist.sys.language=en -prop persist.sys.country=US -avd hudson_en-US_240_WVGA_android-16_armeabi-v7a_1 -no-snapshot-load -no-snapshot-save -no-window -no-audio -gpu off
  $ /opt/android/android-sdk-linux/platform-tools/adb connect localhost:5937
  connected to localhost:5937
  [android] Waiting for emulator to finish booting...
  $ /opt/android/android-sdk-linux/platform-tools/adb -s localhost:5937 shell getprop init.svc.bootanim
  error: device offline
  $ /opt/android/android-sdk-linux/platform-tools/adb connect localhost:5937
  $ /opt/android/android-sdk-linux/platform-tools/adb -s localhost:5937 shell getprop init.svc.bootanim
  $ /opt/android/android-sdk-linux/platform-tools/adb connect localhost:5937
  $ /opt/android/android-sdk-linux/platform-tools/adb -s localhost:5937 shell getprop init.svc.bootanim
  $ /opt/android/android-sdk-linux/platform-tools/adb disconnect localhost:5937
  $ /opt/android/android-sdk-linux/platform-tools/adb connect localhost:5937
  $ /opt/android/android-sdk-linux/platform-tools/adb -s localhost:5937 shell getprop init.svc.bootanim
  $ /opt/android/android-sdk-linux/platform-tools/adb connect localhost:5937
  $ /opt/android/android-sdk-linux/platform-tools/adb -s localhost:5937 shell getprop init.svc.bootanim
  $ /opt/android/android-sdk-linux/platform-tools/adb -s localhost:5937 logcat -v time
  $ /opt/android/android-sdk-linux/platform-tools/adb connect localhost:5937
  [android] Emulator is ready for use (took 59 seconds)
  Copied 3 artifacts from "Android Compile (DEV)" build number 165
  [1] $ /bin/sh -xe /tmp/hudson8490616504917756558.sh
  + date
  Fri Jun 5 10:37:35 PDT 2015
  + adb devices
  List of devices attached
  emulator-5936 device
  localhost:5937 device
  ===end of copy & paste ===

I believe in older plugin, it used the emulator-port convention, but newer build uses localhost:port and somehow this caused adb to think there are two different devices.

Is this issue with this plugin? Or should really be issue with adb?