Import assets and merge with already existing assets

[ServicedeskAT]

We have a number of assets that have already been seen / scanned by Lansweeper.
Now we want to add some additional informations to these assets in Lansweeper (custom fields etc.).
We created a csv file with these assets which included both assets informations already known by lansweeper and some new informations.
When we imported the file using the "Import assets" functions all the assets had double entries in Lansweeper, i.e. there were created new entries with the new informations for the assets (with different domain names).
After some scans and manuel rescans the entries were merged (but not for all assets).

My problem is that I don't understand Lansweeper's behaviour when I use "Import assets".
What do I have to do to get all the newly imported informations merged to one entry per asset?

[DavidPK]

Lansweeper updates existing asset records in its database when scanned data matches. If a match isn't found, or the scanned data cannot be associated with an existing asset, a new record is created. To prevent duplication, Lansweeper uses unique identifiers. For Windows, these are the NetBIOS computer and domain names. For non-Windows devices, the MAC address is used. When importing asset you need to ensure that the fields match or there will be duplicate records created. Why some of the assets merged after is because there was an existing record found when scanned.  

 If these do change, an additional feature, rename detection(Windows only) helps prevent duplication. More information can be found in the articles below:
 

https://community.lansweeper.com/t5/scanning-your-network/windows-rename-detection/ta-p/64311