When the item contains a product dimension like a configuration or style, the warehousing app requires that the bar code may not be same as the item number. This should be possible, because you have scenario's that an item contains always one active configuration. This would also mean that in case of an existing warehouse (not using Dynamics yet), you have to replace all the existing bar codes on the items. Beside that, it is strange that Dynamics allows a bar code which is the same as the item number, but the warehouse app doesn't allow it, should be consistent.