What is wrong with the following sentence (other than the obvious heartbreak): “The thing is, is John just doesn’t love me anymore” ?
It’s the fake relative pronoun. A relative pronoun, obviously, is a word that begins a relative clause. In the sentence just previous to this one, the word is “that;” essentially, this pronoun melds two sentences, “a relative pronoun is a word” and “the word connects two relative clauses,” into one. Other such pronouns in modern English usage are who, whom, whose and which, as you no doubt know from grammar class.
Correctly, the opening example should be “the thing is that John doesn’t love me,” or merely the invisible “the thing is, John doesn’t love me.” Over the years, however, I’ve noticed people inserting an extra “is” in sentences like this because their brain tells them (assumedly) that something is missing from the more natural, more colloquial relative clause “, John doesn’t love me.” In many languages, one needs a relative pronoun at all times; it is not optional (C’est que John me n’aime plus) like it sometimes is in English. This option appears to cause confusion. Should there be something more? What if we just repeat “is”?
In the end I can’t pretend to know the motivation of the human brain, but I do find this phenomenon interesting. I predict that in the future, we may begin to see “is” labeled as a dialectical relative pronoun, much like the “what” found, for example, in archaic rural outposts of the UK: “The boy what eats more meat gets more dessert!”
As a side note, from the quick search I did on the internet, I did not see any research on this subject. I’m totally calling it, then.
Botkin Creative 
Hee. I like this, Kates. Though I have to admit to not having heard this particular construction recently, I think you’ve totally called it. Grammar on.
The thing is, is you should tag Kurt Queller when you write notes like this. He goes wild for this stuff.
But seriously, I’ve got another possible explanation. People like to say “The thing is” and then pause, because what follows is often a Really Big Deal. That’s why there’s generally a comma after “is.” But that makes people think “The thing is” should be a unit, some kind of unanalyzable complex subject. So if “The thing is,” is the subject, and “John doesn’t love me anymore” is the predicate, then obviously something else is needed in between. A linking verb (“is”) fits the bill nicely.
Or maybe the truth is some combination of your idea and mine. You’re the one who got dibs on the whole concept anyway.
Jeff, you’re right, that makes sense also. But how in the world would one test such premises?
I suppose one could ask Kurt…