Office documents with legacy file types (i.e. Microsoft Office 97-2003 .doc, .xls, .ppt) get multiple document IDs when saving to SharePoint through DMS, native Office or directly through the browser interface.
The root cause is that when the document has a Doc ID custom property (_dlc_DocId), SharePoint demotes this value into the list item’s properties prior to assigning a Document ID.
If the value of the Doc ID custom property consists of numbers only, then the list item property is created as an integer, rather than as a string.
When the SharePoint Document ID service attempts to assign a Document ID, it attempts to retrieve this list item property as a string and fails with an InvalidCastException (because the property is an integer).
The result is that no new Document ID is assigned and the Document ID associated with a different custom property instead (_dlc_DocIdItemGuid).
There is also a corresponding entry in the SharePoint logs, under the “Document Management” category:
The current workaround is to include a non-numeric value in the UDN numbering format, so SharePoint recognizes the value as a String, rather than an integer.
See the Numbering Format section in the example below: