Unfortunately, I've never seen this behaviour working since it was added. I use the Jabber plugin.

Normally I use Jenkins 1.409.1, but I can also reproduce this now with a clean Jenkins instance (mvn hpi:run from jabber-plugin, i.e. Jenkins 1.391).
No matter what I set the "Notify for parent projects" value to, I get the same results:

Project matrix » foo build #5: SUCCESS in 0.13 sec: http://localhost:8080/job/matrix/./name=foo/5/
Project matrix » bar build #5: SUCCESS in 0.16 sec: http://localhost:8080/job/matrix/./name=bar/5/
Project matrix build #5: SUCCESS in 1 sec: http://localhost:8080/job/matrix/5/

With the PARENT option selected, I would expect to only see the last line, and not the two individual configurations.

Code changed in jenkins
User: Christoph Kutzinski
Path:
src/main/java/hudson/plugins/im/IMPublisher.java
src/test/java/hudson/plugins/im/IMPublisherTest.java
src/test/java/hudson/plugins/im/MatrixNotificationTest.java
http://jenkins-ci.org/commit/instant-messaging-plugin/d85d10afa0afedf2362d61587966e19235863f4f
Log:
[FIXED JENKINS-8923] make number of notifications for matrix jobs configurable

Strange, I'm pretty sure that I tested it, when I had implemented it first. Maybe some core change broke this.
Fixed in v1.19 of the instant-messaging plugin

Integrated in plugins_instant-messaging #90
[FIXED JENKINS-8923] make number of notifications for matrix jobs configurable

Christoph Kutzinski :
Files :

• src/test/java/hudson/plugins/im/IMPublisherTest.java
• src/main/java/hudson/plugins/im/IMPublisher.java
• src/test/java/hudson/plugins/im/MatrixNotificationTest.java

Works great in version 1.19, thanks!