When configuring an interface with netmask of /32, (using with VirtualNetDevice and tunnels), and Ptr<Ipv4>->Setup(i) is called, this, in turn calls NotifyInterfaceUp() of all routing protocols.

In e.g. 

void
Ipv4StaticRouting::NotifyInterfaceUp (uint32_t i)
{
  NS_LOG_FUNCTION (this << i);
  // If interface address and network mask have been set, add a route
  // to the network of the interface (like e.g. ifconfig does on a
  // Linux box)
  for (uint32_t j = 0; j < m_ipv4->GetNAddresses (i); j++)
    {
      if (m_ipv4->GetAddress (i,j).GetLocal () != Ipv4Address () &&
          m_ipv4->GetAddress (i,j).GetMask () != Ipv4Mask ())
        {
          AddNetworkRouteTo (m_ipv4->GetAddress (i,j).GetLocal ().CombineMask (m
_ipv4->GetAddress (i,j).GetMask ()),
                             m_ipv4->GetAddress (i,j).GetMask (), i);
        }
    }
}

the network is added even when the mask is /32. This leads to same entry being in routing table twice (both the if-addr and as "network", even if the "network" size is /32!). 

Even in Linux, if you add an ip address with mask of 255.255.255.255, nothing gets added to routing table.

Proposed line to add:

if (m_ipv4->GetAddress (i,j).GetMask()==Ipv4Mask::GetOnes()) 
  { 
    return; 
  }

However, not submitting a patch yet - should this be added to ALL routing protocols, or perhaps just list-routing?

In Linux (just a quick test with my wireless interface that I'm not using right now, also omitting lines pertaining to my wired Internet connection). 

# ifconfig wlan up
# ifconfig wlan 4.4.4.4 netmask 255.255.255.255
# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo
# ifconfig wlan 4.4.4.4 netmask 255.255.255.0
# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
4.4.4.0         0.0.0.0         255.255.255.0   U     0      0        0 wlan
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo