When a duplicate key is encountered, the error message is constructed in print_keydup_error:
The duplicate value is silently truncated to 64 characters. This is because the format string for the error message sets the field width to 64:
Truncation is probably fine, but since this specific error involves a "duplicate key", it can be a bit confusing if the user searches the table for that string, and then doesn't find another row with that specific value.
If we can't print the whole value, then the error message may want to mention that the string may be truncated to 64 characters.