Hello Panagiotis Basakidis,
1) in this KB, it is assumed that the field Original DocID is already filled with values. In the KB example, the Original DocID flags duplicate documents - any document which has a value which equals that of at least one other document in the file cabinet, will be flagged as duplicate in the subsequent steps.
You do not have to use Original DocID in your scenario - instead use any other field which can be used in your file cabinet to identify if a document is stored multiple times. You can also use a combination of multiple fields.
2) Your assumption about how the DocID works is correct. The combination of file cabinet and DocID can always be used to uniquely identify any single document. In the database filecabinet table, this number is the primary key.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant