[JIRAALIGN-1352] Custom Fields: User is allowed to create Custom fields with same titles

Issue Summary

Custom Fields: User is allowed to create custom fields with the same titles

Steps to Reproduce

1. Login
2. Go to Administration> Details Panel Settings.
3. Try to create Custom fields with the same titles for the same work item level.

Expected Results

User is not allowed to create Custom fields that have same name. Furthermore, such configuration results in broken Import, User is not able to import file that contains Duplicated columns for custom fields.
Furthermore, #2, renaming of custom field results in broken export, the user is not able to open the file.

Actual Results

User is allowed to do this.

Workaround

The user can manually remove the duplicate column in the objectives tab of the import file.