When you’re creating columns either the Site Columns or the List Columns in SharePoint, suddenly it might show a message stating “The column name that you entered is already in use or reserved. Choose another name.” Here is the fix for this issue.
1. Goto Site Actions > Site Settings > Modify All Site Settings
2. Under Galleries, choose Site Content Type
3. Select the content type and choose the column
4. Edit the column in new window
5. On your browser address bar, type the following and enter:
6. Be sure you get the 'Successfully cleared forbidden columns' alert before proceeding, if not then something is wrong.
7. Change the column name and Hit OK
I sort of got frustrated with this as I could not return back the column name to its original settings but then thanks to this article as I was relieved from any trouble.