To resolve this situation, you will need to make sure the subdomain is all lowercase letters.

To do this, do the following: 

1. Go to Communities > Special Events > Friends Asking Friends > Kintera Thon
2. Highlight your Thon event name, Select Website Info
3. Under the "Event Website Creation Checklist" section, for the Friendly web address, click the Edit button
4. Edit the content for the third text box to the right to make sure all the text is lower case.
5. Click Submit
6. Click Publish This Event Now.

Note: Your Participants should no longer see the subdomain duplicate in the Friendly URL textbox.