Is this to display in a list?
What I would do is first make sure you have a Row ID column. Then create a multiple relation that links the item name to itself. Using that relation, create a rollup column that sums the total quantity for each item. Then create a Single Value column that grabs the Row ID value from the relation (This will be the first matching row ID for that item name). Finally create an IF column then checks if the single value row ID matches the regular row ID. If it does, then return the rollup quantity. Now you can filter you list by checking if that IF column value is not empty, and it will provide a list of unique items and their total quantity count.