This phrase drives me crazy!
The thing is is that people sound so silly when they begin a sentence this way that I have a hard time focusing on their actual point.
Beginning an utterance with "The thing is is that" (sic) is common, at least regionally1, in spoken American English. Although I avoid nurturing pet peeves, I find it impossible to ignore this particular one in conversation. When a sentence begins this way, the speaker comes across as one who does not think about what he says, or more to the point, one who expects her meaning to be discerned in spite of her words, rather than through them.
What is it supposed to mean?
A likely explanation is the accidental mingling of two common sentence beginnings: "The thing is that ____" and "What it is is that ____". While not especially great ways to start a sentence, they are grammatically OK. "The thing is is that ____" is not
What is better practice?
Try variations like these. Note, it's not the presence or absence of the phrase "the thing" that makes it right or wrong, though it's always better to substitute something more definite in place of "the thing".
- "The issue is that this government's on a highway to Hell."
- "Their problem is that they don't understand macroeconomics."
- "My point is that you stole my kidney and I want it back NOW."
Any of these formulations can be sensibly understood. Now what if you've already begun the sentence, you're part of the way through it, and you hear yourself say "What xyz is," ... now you're committed to that clause and you need a way out. In this case as long as you began with "What", it's all right to continue with "...is that abc". But if you did not begin with "What", if you just started off with "The thing is", or "The problem is", etc., then your best bet is to continue with "... that abc". This is not perfect but it's far better than doubling the word "is" for no reason.
What's the problem, then?
The sequence "What ___ is" can be followed by "is that". It is pointing out an equality between the contents of the blank and whatever comes after "is that". On the other hand, the sequence "The thing is" cannot be followed by "is that". "The thing is" is not a noun or a variable that can be considered equal or comparable to something else. One must either add a What at the beginning, or remove the second occurrence of is.
What to avoid, in one easy step
The core of this speaking habit is the sequence "is is that". Think about it; there is only one way to have this sequence of words make sense in a sentence — to immediately precede it with What x, where x is a noun. No other usage of "is is that" makes sense. Rather not have to remember a specific rule like that? Good! Best practice is to simply never utter the words "is is that" in that order. It is thereby easy to weed out the habit and leave it behind.
1 It seems to be regional — I haven't fully worked out the distribution. I hear it regularly in the western U.S., but have also noticed it coming from other regions. I'm interested to hear from readers who have heard it elsewhere.