Hi Benedikt,
You may have hit on a bug here. I am not an expert in volume transmitter matters, but a quick look through the code reveals that volume_transmitter::update() stores spike multiplicity here
https://github.com/nest/nest-simulator/blob/47b7e43f72317911b3e285daa9ecd4f4...
but that stdp_dopa_connection::process_dopa_spikes_() does not read the multiplicity information in its dopa_spikes argument:
https://github.com/nest/nest-simulator/blob/47b7e43f72317911b3e285daa9ecd4f4...
Could you create an issue on Github, ideally providing a minimal reproducer/test for this issue?
Multiplicity handling between neurons is discussed in this issue
https://github.com/nest/nest-simulator/issues/77#issuecomment-148995091
Multiplicity is resolved properly in any case when "normal" neurons send spikes to each other, because those spikes are transmitted via send_remote(), which always unrolls multiplicity. Spike trains from devices (any network node class for which has_proxies() returns false) send spikes with multiplicity. So if you connect, e.g., a poisson_generator directly to a volume_transmitter, multiplicity will be ignored if my reading of the code above is right. If you put a parrot_neuron between the poisson_generator and the volume_transmitter, multiplicity should be handled correctly.
Best, Hans Ekkehard