Having a duplicate custom field name breaks the Notify Slack post-function renderer and the notification

Issue Summary

• Jira Slack integration throwing error when duplicate custom field names exist
• Affecting Jira Server for Slack version 2.0.1.

Steps to Reproduce

1. Configure a Slack integration in Jira
2. Create duplicate custom field with the name 'Category' ( Other duplicated custom fields are affected too)
3. Configure the Notify Slack post-function

Expected Results

Renderer works fine and notifications are sent correctly.

Actual Results

• The notification preview in Notify Slack post-function throws the following error:

  Error rendering preview
  Duplicate key JiraPostFunctionEventRenderer.CustomFieldWrapper(field=Category, issue=com.atlassian.jira.plugins.slack.web.rest.SlackMessageResource$1@432a38d8)
  

• The notification in Slack breaks with the error

  Error in Notify Slack configuration for project XXX. Please contact your administrator to make sure that Notify Slack post-function comes after the issue creation.

Notes

The plugin pre-populates values of all custom fields for message rendering, thus it is breaking the rendering with the Duplicate key error.

Workaround

Do not use custom fields with the same name
OR
Revert to the add-on's version 1.1.11 which is not affected from here