Adding a WHERE clause is no option for two reasons:

1) It only works for INSERT...SELECT...WHERE... statements. If INSERT...(<field list>)VALUES(<value list>) is used, the WHERE clause is not available.

2) INSERT...SELECT...WHERE... may insert multiple values at once, all of which are not yet present in the table, so the WHERE clause will give you no restrictions. Still within the inserted values there may be duplicates which will be inserted later unless you enforce uniqueness by means of an index as well - and then the WHERE clause was pointless and creating nothing but overhead.

At least it is just a single point I lost... 🙁