If a protocol like SNMP or SSH is enabled, network devices with multiple IP addresses shouldn't generate multiple assets. From your initial post, I thought the issue was simply multiple scan requests being performed for the same device, not multiple assets being generated for the device.
If a network device has multiple IP addresses and the SNMP protocol is accessible for all IPs, Lansweeper will be able to pull a full list of device MACs from all IPs. Based on the overlap in MAC addresses, Lansweeper will know all of the IPs belong to the same device and only generate a single asset. I assume your network devices don't have SNMP enabled, in which case the ideal solution would be to enable this protocol.
Linux, Mac, Windows and VMware machines with multiple IPs shouldn't generate multiple assets either, if scanned through SSH, WMI and MOB, respectively.