With a subquery which counts the number of assets having one specific user as last logged on user you can get this list:
Select Top 1000000 tblAssets.AssetID,
tblAssets.Username,
tblAssets.Userdomain,
tblAssets.AssetName,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssets.Lastseen
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Inner Join (Select tblAssets.Username,
tblAssets.Userdomain,
Count(tblAssets.AssetID) As number
From tblAssets
Group By tblAssets.Username,
tblAssets.Userdomain
Having Count(tblAssets.AssetID) > 1) tCount On tCount.Username =
tblAssets.Username And tCount.Userdomain = tblAssets.Userdomain
Where tblAssetCustom.State = 1
Order By tCount.Username