Magento 2 - Item (Magento\Catalog\Model\ResourceModel\Eav\Attribute\Interceptor) with the same id already exist
The error 'Item with the same ID ... already exist' happens when Magento tries to load a collection with an SQL query which does not return distinct rows.
You need to find out where the collection is being loaded and what SQL query is being used in order to start debugging the problem.
In more detail:
When Magento loads a collection, it uses the ORM design pattern. Briefly, this means that an SQL query is run and every row of the result set is turned into an object. By 'turned into' I mean a new object is instantiated and then the columns of the result row are added as properties to the object. These objects are then added to an instance of the Collection class. The error you're experiencing is because Magento checks that each object in the collection is unique, i.e. That the ID of each object is unique. If it isn't, then Magento throws the 'Item with the same ID ... already exist' error.
If this is a part of Magento you've not modified, then it is most likely an extension or a theme which is using a plugin or event/observer in order to modify the query for the collection.