Can I sync an existing database using a .dmp file?
Next time, instead of "deleting everything", you can tell imp to ignore table creation error and still load the data by specifying the IGNORE=y option:

imp user/password file=MyExportFile.dmp IGNORE=y


There are few other considerations that you should make when importing into an existing schema, as outlined in the manual. Specifically, you may need to disable referential integrity constraints if they exists, otherwise some data may be rejected.