Could you add an assertion like that at the end of AudioProcessor::addParameter()
?
#ifdef JUCE_DEBUG
String paramId = getParameterID (managedParameters.size() - 1);
for (int i = managedParameters.size() - 1; --i >= 0;)
jassert (paramId != getParameterID (i));
#endif
Or something equivalent somewhere else, just to check that there is no 2 different params with the same id.