We have a client here in Minnesota that reported having duplicate item numbers in Microsoft Dynamics GP. Knowing that the item number is the primary key of the item master table (IV00101.ITEMNMBR), I knew this could not be true. So there must be some invisible character.
We did find that after the item number an additional ascii character existed, character #160, which is the non-blank space (nbsp from HTML). A user must have copied and pasted an item number from a webpage that included a nbsp character after the item number, and GP allowed this action.
To correct the issue, we changed the item description for the item with the char(160) to be "Invalid". Then, we used the GP Professional Service Tools Library's Item Combiner tool (which is now free to current GP customers) to merge the "Invalid" version of the item into the valid version of the item. Problem solved!