As just discussed on IRC:  notification sounds for "highlight triggered" and "nick written"  (and maybe others) are broken, as in: they either play twice or not at all. Preview works, can be reproduced with both phonon-gstreamer and -vlc. 

As requested, the Log:

[19:20:25] <Fuchs> There is a somewhat annoying bug in konversation, could you do me a favour and try to reproduce it? 
[19:20:39] <Fuchs> All notification sounds are played twice :( 
[19:20:44] <Sho_> Fuchs: I'm a bit pressed for time (dinner coming up) but if it's not too involved, sure?
[19:21:06] <Fuchs> if you can reproduce that: it would be lovely to see it fixed. Konvi is current git, phonon is both gstreamer and vlc, other applications don't do that, 
[19:21:21] <Fuchs> happens both with  "nick written" and "highlight triggered", and also if I disable one of them and only have the other
[19:22:20] <Fuchs> and I also tried with a fresh konvi config (removed both the notifyrc and konversationrc) 
[19:23:28] <Sho_> Fuchs: use my nick in a few moments
[19:24:22] <Fuchs> Sho_: as in: now? 
[19:24:33] <Sho_> hmm
[19:24:37] <Sho_> well that was no sound at all
[19:24:44] <Fuchs> try "nick written", 
[19:24:48] <Sho_> i did
[19:24:50] <Fuchs> highlight triggers sometimes doesn't do it 
[19:24:53] <Fuchs> okay, that is odd 
[19:24:54] <Sho_> preview worked, too
[19:25:07] <Fuchs> Anyway, I don't want to stress you. Shall I open one on bugs.kde.org, so you can track? 
[19:25:46] <Testkonv> Sho_: make some noise
[19:26:07] <Testkonv> Sho_: make some noise
[19:27:19] <Testkonv> Sho_: hein
[19:27:27] <Testkonv> Sho_: hein
[19:27:35] <Testkonv> Sho_: make some noise
[19:27:37] <Testkonv> hrm
[19:27:53] <Testkonv> Fuchs: so I think I know what's going on
[19:28:02] <Testkonv> Fuchs: i get no sounds played and yours play twice
[19:28:02] <Fuchs> Testkonv: lovely
[19:28:06] <Testkonv> obviously you stole my sound
[19:28:10] <Fuchs> Testkonv: of course
[19:28:33] <Fuchs> Anyway, now that you are aware of it I am sure it will be fixed, since you are awesome
[19:28:41] <Fuchs> now I shall take care of my dinner, and probably you of yours :) 
[19:29:17] <Testkonv> Fuchs: file a ticket tho please
[19:29:22] <Fuchs> jus
[19:29:25] <Testkonv> no idea why my sound doesn't work in reality
[19:29:28] <Testkonv> Sho_: make some noise
[19:29:34] <Fuchs> I have no idea what to write though, but I'll do my best
[19:29:47] <Testkonv> Fuchs: if in doubt just dump the log or so
[19:29:52] <Testkonv> i mostly need a reminder after all

Reproducible: Always

Steps to Reproduce:
1. Configure notifications to play a sound on nick written
2. Open a second connection
3. Write your nick
Actual Results:  
No sound / sound is played twice

Expected Results:  
Sound is played exactly once

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