Can you reproduce this error on other existing or new content types?
You can use this PowerShell script to find the column with same Internal Name, give a test with the out of the box columns as well.
Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue
$web = Get-SPWeb "http://sharepoint.crescent.com/sites/operations"
You either have an internal name or GUID conflict with another column being added to this list. Try removing all of the columns and adding them back one by one to find your problem child.
Here are some other references that may help:
Cannot add new column - "Field name already exists"
Field name already exists
This is (a backup copy of) the article by Tyler Holmes which is mentioned by Marius Constantinescu. I have used it a couple of times and I would hate to see it disapear...
DELETING CONTENT TYPES IN SHAREPOINT 2007: A TROUBLESHOOT
[Updated Nov, 13 2008]
Whenever it comes time to delete a content type, it's either a 10 second job or a multi hour why-did-I-...
You can hide the column from all forms by following below steps:
Go to List settings --> Advanced settings.
Set Allow management of content types? to "Yes" and click "OK".
On List settings page, Click on content type name under "Content Types"
Click on column name under "Columns" section.
Set Hidden (...