One of the characteristics of the book of Daniel is the recurrence of themes already introduced.
A first vision presents a general architecture.
A later vision returns to certain elements, develops them or clarifies them.
This method appears particularly in the visions concerning the kingdoms.
It is therefore preferable to read the prophecies according to a principle of progression and deepening rather than as totally independent revelations.
This observation does not mean that all the visions describe exactly the same events.
Some have a broader scope; others focus attention on a particular period or question.