So, assuming you store the user's information in your database on the first registration attempt, all you need to do is select from that same table with a where clause that identifies the host and a range of timestamp values. In Oracle, that would be something like this:

"SELECT * from USERS where host = ? and date >= (sysdate - 0.00139)"
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If you get any rows, then someone has tried to register from that same host in the last 5 minutes. If you just want to prevent the same username, add 'username' in the where clause. By the way, that's a really bad name for a date column.