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When you manually assign a duplicate IPv6 address with only a difference in the case of the letters (for example, fd00::10:20a is already existing and you try to assign fd00::10:20A to another adapter), you are prompted to restart the computer to take effect. This may give the impression that the duplicate address becomes valid after restarting the computer, when in fact the duplicate address is never set because IPv6 address is not case-sensitive and address duplication is not allowed.
Steps to reproduce the problem
To reproduce the problem, follow these steps on a computer that has more than one network adapter.
1. In [Control Panel], click [Network and Internet] and click [Network and Sharing Center].
2. Click [Change adapter settings] on the left pane.
3. Right click on the connection icon and open [Properties]
4. Double click on [Internet Protocol Version 6 (TCP/IPv6)] and input "fd00::10:20a" for [Use the following IPv6 address]
5. Click [OK] on [Internet Protocol Version 6 Properties] dialog, then click [OK] on [Local Area Connection Properties] dialog.
6. Repeat the steps above for another connection but with the duplicate address "FD00::10:20A" at the step 4.
After setting the duplicate address for the second adapter, you receive the following message.
You must shut down and restart your computer before the new settings will take effect.
Do you want to restart your computer now?
However, restarting the computer will never make the new IPv6 setting take effect.
Here is the right message for this situation.
The IP address ****** you have entered for this network adapter is already assigned to another adapter 'xxxxxxx' on this computer. If the same address is assigned to both adapters and they both become active, only one of them will use this address. This may result in incorrect system configuration.
Do you want to fix the problem by entering a different IP address for this adapter in the list of IP addresses in the Advanced dialog box?
Be aware that upper case letters are treated as the same letters as their lower case counterparts for IPv6 address and make sure to assign a different address to each adapter.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
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