Something's got confused

Something's got confused

We have several hundred workstations in our Asset list that are fine, but we have a strange problem with a couple.
 
These 2 workstations are named WS1 and WS2 and are used by User1 and User2 respectively. They are identical HP PCs running Windows 7, and both are working fine. However, I can't get both of them to appear on the Asset list at the same time.
 
If I scan WS1, it appears as an asset, showing that it is assigned to User1 and last logged on by User1 (which is correct). If I then scan WS2, it appears as an asset, but assigned to User1 and last logged on as User1 - but WS1 has now disappeared.
 
I corrected WS2 so that it is assigned to User2, then reran a scan on WS1 - this comes back (now assigned to User2) but WS2 disappears.
 
So somewhere behind the scenes, SD seems to see these two PCs as linked in some way. Is there a way to remove all reference to both WS1 and WS2 and start again - I've tried just deleting them both but the same problem occurs again.

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