I don't think that it can. Definitely possible to get a duplicate. Statistically impossible though. On the same machine then supposedly impossible because algorithm includes time (although I am not sure that is the case for new formulae). But, assuming time included, what about for the hour when the clocks go back in the Fall? Or some hardware issue with the clock ... or it being reset a smidgeon by SYNC with Time Server ...
GUIDs generated by TWO machines then definitely (albeit rate) evidence of duplicates.
On computers with poor implementations of GUID algorithm ... chances increase
Like I said, it is not something I worry about, but we do put a Unique Index on GUID columns just-in-case. If we have a GUID column we will likely be retrieving records based on it, so the Index will come in handy anyway, and the SQL Query Planner will appreciate the Unique attribute