Grammar – Accepts vs Accept: Understanding Proper Usage

For technical documentations, you can sound as sophisticated as you want: if the reader gets immediately what he/she is supposed to do, it doesn't matter.

I would however follow the advices of “The Elements of International English Style” by Edmond Weiss, as they refer to a more "neutral" form of English, which has greater chance to be understood by any reader.

• write with sentence simple enough to understand without any ambiguity (i.e., the Principle of Simplicity).
• write with sentence clear enough to understand without any ambiguity (i.e., the Principle of Clarity).
• reduce the visual burden of reading due to lazy punctuation, page layout, or a combination of other factors (i.e., the Principle of Reducing the Burden).

The last chapter addresses the inescapable need for cultural adaptation.
Even if one is a master of English he or she can still make errors in technical documentation of the cultural context is ignored.

Doubtless some will say NGrams isn't appropriate here, but I think this is interesting...

Obviously many of those instances are "false positives" (particularly, the earlier ones). But the increase in prevalence is quite marked, and glancing through a few pages of the later instances shows that many of them are indeed the construction OP asks about.

Structurally, what seems to be happening is that two separate elements (for example, "the thing is" and "is that") have both become grammaticalised in the mind of the speaker, so they're seen as independent self-contained grammatical items - each containing its own copy of the word "is".

As the chart suggests, it's very much an emerging usage that's gradually extending its scope, so...

"What it is is that blah blah" - seems unremarkable to me.

"The point is is that blah blah" - sounds somewhat "off", but I can live with it.

"The problem with this is is that it sounds weird" - to me, at least, it really does.

Different speakers will draw their own line as to where the construction becomes "unacceptable". Some people may think it makes a difference whether there's a comma between the two instances of "is". But because this is (still?) primarily a spoken usage, and people don't punctuate speech, I think that's largely irrelevant.

Addressing OP's specific question, I'd advise against using the form in "professional" writing for the two reasons mentioned above - it's mainly a spoken rather than written usage, and not everyone will be happy with it in any given context.