We would like error handling to be consistent across load types.
For promotions, if duplicate items are included in the bulk load, only the second, third, etc. instances of the bulk loader fail.
For relationships, all instances of the duplicate record fail - the relationship is not added.
In the example below, lines 2 & 3 were duplicates and line 4 was unique.
Neither line 2 or 3 was successful. In a promotion instance, line 2 would be successful and line 3 would produce an error.

See Zendesk ticket #168693