Steps to reproduce the issue

• Go to administration of Content component
• Create a category called A
• Create a category called B and select parent category to A so the category tree looks like this: A -> B
• Create custom article field called Test and as a category select both A and B. Then in Options tab select Automatic Display to Before Content
• Create article called Test Article in B category and fill Test field with with: This is the test field
• Create a new articles blog view menu item displaying articles from B category
• Go to front end and open the new menu item you created
• Find the article called Test Article and open it.

Expected result

This article Test field should only be visible once

Actual result

Field is duplicated

System information (as much as possible)

3.9.4 but I'm pretty sure it goes back to the introduction of custom fields

Additional comments

I investigated the matter and it is caused by incomplete query in FieldsModelFields model (method getListQuery) from /administrator/components/com_fields. There is no group by in the query so field will be duplicated as many times as the number of parent categories on which your custom field exists. So if your tree is like A -> B -> C -> D and you place the article in D with setting the custom field to be available in all 4 categories you will have this field displayed 4 times. Simple group by should do the trick. It exists in content component but I'm pretty sure it affects all components using custom form fields and categories.