If you have an item with inventory in more than one location, and move some of the inventory into a location that already contains inventory for that item, it will show up on separate lines on the List Inventory screen.
To correct this, download and extract the attached files on the server, then run UpdateDB.bat. (If you have Enterprise, you will need to edit RemoveExtraInventoryLines.wql with Notepad first to enter the correct instance name and sa password)

This will correct the issue with your current inventory, but until a patch is released to correct the cause of the issue, it will keep occurring. Retain the files so you can run them again as needed.