ExoDrone was originally released as a Warchip unit and its ability required 3 Exosoldiers instead of the 3 Robotic units it requires now. This caused an uproar within the War Metal player community as Exosoldier was released months earlier and was no longer available. The developers responded and relaxed its ability requirements to 3 Robotic units. To compensate for its increased usefulness, it was transformed into a Collection unit.
When the Archive was introduced, ExoDrone was accidentally included as a purchasable War Chip item. It was not purchasable if you already had ExoDrone as it is a Unique unit. If you purchased ExoDrone from the Archive and then completed the ExoDrone Collection, you would still only have one ExoDrone.