Joining two overlapping databases

Hello,
I had a user deleted and would like to put all of that user’s data back in from a mysql dump I have from the night before that happened. The problem is that there are a lot of duplicate entries in the old sql dump and the current database. Is there a way to ask it to put them in but to ignore or overwrite duplicate entries? As it is, I get a table already exists error when executing: mysql -u username -p dbname > old_backup.sql

Thanks in advance!