When duplicating module the course cache is rebuilt twice

1. Duplicate last module in a section
2. Refresh course page and make sure the duplicated module is still in the correct position
3. Duplicate another module (not the last one)
4. Refresh course page and make sure the duplicated module is right below the original module

If you want to test performance win:

1. Create a course with many modules
2. Open course page in editing mode
3. Add 'logall' => 1 to the $CFG->dboptions in the config.php
4. Duplicate a module
5. Count the number of new records in mdl_log_queries table
6. Revert the commit
7. Duplicate a module
8. Count the number of new records in mdl_log_queries table - it is much larger (the bigger the course the larger is the difference)
9. Remove 'logall'