How to read this output from MySQL?
Run this command : SHOW CREATE TABLE log_url\G

This will give you a display of the SQL needed to create the table.

It will also show you the indexes and constraints for that table.

The first index (key 1) in that table must have a column called url_id.

From the error message, you are being told that there exists a row in log_url that has 41721 already present. You can verify this with SELECT * FROM log_url WHERE url_id = 41721;