I thought it was incorrect to say I myself as in:
I myself don’t like this idea.
However, last night I was watching the second Harry Potter movie, and one of the characters said:
In case you ever need to defend yourselves as I myself have done on countless occasions.
I think there might be a difference between the two, but I wonder whether the first one is correct or even whether the second might be wrong.
Myself is a reflexive pronoun. It’s called that because one of its uses is to reflect the action of a verb back onto the subject, as in ‘I’ve hurt myself’. Yourselves is used in the same way in the sentence you quote from ‘Harry Potter’.
Reflexive pronouns are also used for emphasis, and that is how myself is being used in ‘I myself don’t like this idea’ (although that would probably occur as ‘I don’t like this idea myself’). That is also how it is being used in the second part of the ‘Harry Potter’ quotation.