What is the non-standard grammatical feature in this sentence? [duplicate]

In the following utterance:

“You know all you’re getting off it then is maybe the CD-ROM which surely that’s not worth grabbing”.

I’m trying to say that they use non standard grammar by using the demonstrative “that” (in the bold) which would normally not be there.

But what is the precise standard ‘rule’ that is not being followed in the bolded text (not the rest of the utterance) hence making it non standard? (Rule is in quotations as I don’t mean to say that it’s necessarily incorrect.)

E.g. In “I didn’t eat no apple” is non standard due to the double negation.

Note that this is spoken, not written.

*You know all you’re getting off it then is maybe the CD-ROM which surely that’s not worth grabbing.

The basic relative clause can be analysed as "which is not worth grabbing", where "which" is subject with "CD-ROM" as its antecedent.

The existence of an anaphoric link between "which" and "CD-ROM means that it is not possible to have a further anaphoric element ("that") linked to the same antecedent, i.e. "CD-ROM".

The speaker's brain has changed course mid-utterance, and everything after which is almost a separate sentence. The "which" means something like ", and the upshot of that is:".