You are correct, we store the information in our database. Someone else might be able to help you with exact information or steps for deleting WIT - it could be a lot of work though given you need to remove all the custom fields and rules for that WIT plus deleting all the work items for that Work Item Type.

I found out from another team member (Sagar) that this could be due to mutliple forms getting added to one work item type.

To fix this issue, you should try to export all the work item types in the project and look for multiple form definition. If you see any WIT definition has multiple form definitions in it then edit the WIT definition to have just one form definition and then import that WIT. Once you have removed duplicate form definition from all the WIT definitions in the database. This error should go away.

Hope this helps!

-Mohammad