[feature] remove duplicated points in synapses

[xiuliren]

some pre and post synapses points are duplicated, we have to remove them.

[xiuliren]

implemented in
#262

We can keep improving it. There exists one issue:
The duplicated post synapse should come from presynapses on the same neuron. This condition was not checked. If they come from different neuron, it should not be considered as a duplicate.