This error is caused by JIRA Software upon creation of a Sample Scrum Project. A field is added to multiple tabs on a screen, which JIRA does not permit. And when you load the story map for one of those projects an error is encountered.
If you see the error "500 - Duplicate key com.atlassian.jira.rest.v2.issue.FieldMetaBean" it means that there is a duplicate field on a screen. To remove it, follow the steps below:
*In case there are mutiple tabs, check for each tab whether the field is called twice
More details can be found in this JIRA Knowledge Base article: https://confluence.atlassian.com/jirakb/createissuemeta-returns-duplicate-key-error-872016885.html