Issue with sharepoint 2003 - ne

Hi,

We have deployed TFS with sharepoint server 2003 and using the single
server setup for document and source code version control. We have
discovered an issue that is causing some trouble in team which allows
any user to checkin a document that is checked out by another user.
Details are as follows:

1. "Checkout" a document using system 1 (user 1)
2. Open the document1 for "Edit in MS Word"
3. Change/update the document and don't save
4. From 2nd system login as user 2 and navigate till document1 folder
5. Right click on document,  notice that user2 is having an option to
6. Click on "CheckIn" (notice that user2 is allowed to "Checkin")
7. Try Saving the document1 using system 1
8. Notice that sharepoint displays a message "This document has already
been Checked In and cannot be saved"

I'm just wondering if there is any workaround or fix to this issue?

Cheers!
D2