Qt warning of type conversion already registered

[nyaruko]

I have a multi-thread Qt program where the Qt sometimes give a warning:

Type conversion already registered from type QPair<QByteArray,QByteArray> to type QtMetaTypePrivate::QPairVariantInterfaceImpl

I have found the warning is from this Qt's function:

@bool QMetaType::registerConverterFunction(QtPrivate::AbstractConverterFunction *f, int from, int to)
{
if (!customTypesConversionRegistry()->insertIfNotContains(from, to, f)) {
qWarning("Type conversion already registered from type %s to type %s",
QMetaType::typeName(from), QMetaType::typeName(to));
if (f)
f->destroy(f);
return false;
}
return true;
}
@
I have checked my own code, nobody is calling this function, so it must be inside Qt's own function.
Could any one give me a hint what's causing this issue or which function actually called the above function inside Qt's code.
I am using Qt 5.3.2.
I wonder whether this could be a Qt's bug instead of mine?