It depends upon your national regulations.
The flight is all logged as DUAL because its instruction i.e. 1 hour.
The logging of IF is incidental to that process, you log however much of that hour that was actual IF. The times are concurrent, not cumulative.
Under JAA rules, the Instructor can only log IF if he is actually flying IF! Pre JAA, FIs often used to log IF when instructing!
Once you have logged enough hours IF to obtain an ATPL (100 hours) then it has little or no value logging any more, except that in the UK The ANO says you must.