In the sentence "Go help yourself", is 'yourself' both the direct object and the subject?

In the sentence "Go help yourself", is 'yourself' both the direct object and the subject?

Can a subject and direct object be the exact same word in a sentence? If not, what is 'yourself' here?

You are almost right.

This sentence is simply an imperative clause.

As in: Go help your sister or Please help your sister.

The subject in these sentences is you-but when we actually say the you, the sentence is more commanding/ emphasized, as in you go help your sister, etc.

the pronoun yourself is a reflexive pronoun and we use it when the subject and the object of the clause are the same, so in a way you are right, though a in your sentence you is the subject

(Someone else answered while I was writing this, but it gives more complete information.)

"Yourself" is a reflexive pronoun functioning as a direct object, used here with an imperative verb. The British Council's Learn English says "We use a reflexive pronoun: as a direct object when the object is the same as the subject of the verb". "Yourself" refers to the subject and object, but grammatically it's the object.

The verb is an imperative form, which doesn't take a subject (hence no subject here), but it's more complex than a usual imperative, which can be one word (e.g. "Die!") or verb plus object ("Hit him!"). "Go X" where X is also a verb is a colloquial equivalent to "go and X" (see under "go and" here) - hence "go fuck yourself" equals "go and fuck yourself"; Macmillan dictionaries gives "go and do something"="used for telling someone to do something, especially when you are annoyed".