I came up with a workaround for this. It doesn't iterate over the matching activities (with the same part number). Instead, I just ignore these matches (Shipping from multiple locations), since I have everything I need in the first instance (original) of the part. Since the original part is the only line item (matching part - matches the other two activities by part number but shipping from another location) that contains a quantity requested, I would include a condition in the 'Search records' step of the Pipeline to exclude anything that has a blank or null quantity requested. Since my original part uses a formula query field that already combines the total quantity shipped of all like parts, I can use that to create my receiving activity. Keep in mind that the 'shipping from' location in my receiving task does not matter on the receiving end as our ordering locations only care that they receive X quantity of parts. They're not concerned about the location from where the part was shipped.
This doesn't solve the technical question of iterating over records with the same part number and somehow combining them, but for this specific dilemma, I was able to create a workaround to the issue.
