You're asking two questions regarding "all there is are idiolects" (the comma is extraneous and will be discarded for this answer), which I'll paraphrase:
- Is "is are" a double copula here?
- Is there a grammatical problem with plurality for "is are" here?
1. Is there a double copula?
Copula
A connecting word, in particular a form of the verb be connecting a subject and complement.
- ODO
The double copula in the sense of "the thing is, is that ..." is ungrammatical. Your idiolects example has no double copula.
In the phrase all there is, the word is is not a copula - it doesn't connect a subject to a complement. It is instead an instance of the existential is. Setting aside the issue of plurality for the moment, the phrase all there is translates to all the instances.
Breaking down the sentence, we have:
- all = everything (noun);
- that: "Used to introduce a defining clause, especially one essential to identification" - definition 5 in ODO (see also example 5.1 in the same link). Constrains all to those that exist (see below);
- is = "exists" - definition 1 in ODO (see the last example in definition 1). Defines which instances of all are being considered;
- are: copula, joining "all that is" with "idiolects"; and
- idiolects: noun
So "all that is" is a noun phrase. Leaving aside the issue of plurality again for now, we can replace that noun phrase with the pronoun "they". The sentence then reduces to, and has the sense of,
they are idiolects.
As you can see, there is no double copula here. Likewise with "all there is is a cat" and "all there are is a sock".
With a cat is is all there, though, "a cat is" isn't a noun phrase. Both instances of is link a cat to all there, so that second version is a double copula and it is therefore ungrammatical.
2. Is there a grammatical problem with plurality for "is are"?
I'm still a little unsure of this part but will articulate an opinion and invite further discussion.
The plurality of all isn't in question - it has singular or plural agreement depending on what it references. See this ELL answer, this EL&U answer, and rule 8 of this link. For example, "all of the roses are red" and "all of the rose is red" are both grammatical. We're then left with whether the noun phrase "all there is" can have plural agreement.
Consider a similar sentence: All that's left are scraps. The word all refers to the collection of things left behind, perhaps a few bones and some gristle. I suspect that if there is flexibility in regarding the collection as either a unit or as a plurality of its constituents, we also have the flexibility to consider the noun phrase as singular or plural. The singular case would give rise to a sentence like All that's left is scrap.
In your example, the singular case would be "all there is is idiolect", but the plural case "all there is are idiolects" is also grammatical. Note, though, that the singular case for both the scrap and idiolect examples raise the level of abstraction - they refer to the nature of the instances rather than the instances themselves.
