Double setParameter call

When I change the selection on my comboBoxes, it usually calls setParameterNotifyingHost() followed by setParameter(), as expected. But sometimes, it then calls an additional setParameter(). Here's the stack trace on the second call:

#0    0x1937e0aa in PrimerAudioProcessor::setParameter(int, float) at /Users/Joe/Developer/Primer/Builds/MacOSX/../../Source/PluginProcessor.cpp:126
#1    0x193b994a in JuceAU::SetParameter(unsigned long, unsigned long, unsigned long, float, unsigned long) at /Users/Joe/Developer/Primer/Builds/MacOSX/../../JuceLibraryCode/modules/juce_audio_plugin_client/AU/juce_AU_Wrapper.mm:507
#2    0x193a09b7 in CMgr_AudioUnitBaseSetParameter(AUBase*, unsigned long, unsigned long, unsigned long, float, unsigned long) at /Applications/Xcode.app/Contents/Developer/Extras/CoreAudio/AudioUnits/AUPublic/AUBase/AUDispatch.cpp:388

Weirder still, this only happens in AU on Ableton. AU in Logic works fine. VST in Ableton works fine. JUCE 2.1.8, Mac os 10.7

I've tried having the isMetaParameter() return true, and false. In both cases the behavior persists. Any ideas?