How to fix out-of-order sequence number with existing GTID error

Question

I am attaching a database to a master and after a while, I get the following error when I run show slave status, I get the following error:

An attempt was made to binlog GTID 1-XXX-XXXXXXXXX which would create an out-of-order sequence number with existing GTID 1-YYY-YYYYYYYYY, and gtid strict mode is enabled.

I would appreciate not just steps on how to fix it, but perhaps an explanation of why this error is happening in the first place.

Thanks!