See myisam_enable_keys-10506 test. Starting from 10.0, MyISAM adds unique indexes using “repair by sort”. Not surprisingly, it cannot correctly detect the conflicting key value. It reports a "duplicate key value XXX", while XXX is not, in fact, duplicate.

Possible fixes:

• use a different error message in this case, the one that does not print the conflicting key value
• detect the conflicting key value correctly, somehow.