That really depends on the design of the database.
Lets say the database is set up as follows
Table : contacts
Fields: id, firstname, surname
Table : phone numbers
Fields, id, type, number
Sample Data contacts:
100, Mr Smith
Sample Data phone numbers
100, home, 0123456
Now... if the contacts ID is an auto generated serial number, when you duplicate the record, you will end up with
300, Mr Smith (300 being the next available serial id)
Since 300 no longer relates to 100 for the phone numbers, they are not shown..
If this is the case, you would need to intercept the duplicate command, using a custom menu perhaps, to run a script similar to one discussed here http://fm.lithium.com/t5/Using-FileMaker-Pro/Copy-Records-with-Portals/m-p/49049#M38360