You need to either:
- Submit the correct computer name *and* domain name on the asset page you manually create. Also make sure the asset type is set to "Windows". Only then will Lansweeper update the existing page when the computer is scanned, as a computer's domain name/computer name combination is its internal ID.
- Enable rename detection in the Lansweeper web console under Configuration/Scanning Setup/Server Options and submit the correct MAC address, model and serial number on the asset page you manually create. Also make sure the asset type is set to "Windows". Rename detection identifies Windows computers based on MAC, model and serial.