Perhaps I've got the wrong idea, but I've been taught that two verbs ending in -ing shouldn't go together. However, I come across such cases occasionally.
*I'm considering buying a new car.
I'm practicing playing the piano.
He is finishing saying his prayers at the moment.*
So, is it grammatically correct to put one -ing after another?
I'm considering buying a new car.
The second verb is not actually a verb, but rather a verbal.
A verbal is a word formed from a verb but functioning as a different part of speech such as an adjective or noun.
Unlike other languages, in English a gerund is a verbal ending in -ing that functions as a noun.
Like an ordinary single-word noun, a gerund may be used as a subject, object, object of a preposition, or subject complement.
Ex.
Running is my favorite form of exercise.
I love running.
I get a high from running.
I’m thinking about running in the marathon.
So your confusion about putting two “ing” verbs together is unwarranted in these types of sentences.
For more information https://webapps.towson.edu/ows/verbals.html
There is a surprisingly large literature on this question, beginning with a 1972 paper from John Ross, Doubl-ing. Here is an extensive discussion from 1999 which is on line: Gerund participles and head complement inflection conditions by Pullum and Zwicky. It has a bibliography of previous work.
The answer by Gandalf is good as a first approximation, but the fact that gerunds are actually verbs could tell you that there is more to it than simply a rule excluding two contiguous verbs in -ing.
The short answer is 'yes'.
"I'm considering buying a new car" is something you'll hear said quite often, and it's perfectly grammatically (see what I did there?) correct.
While "I'm planning going on a diet" is grammatically correct (using an '-ing' verb can work with any otherverb following it is correct), it is not considered good use of English, just as "always isn't" instead of "never" is bad use of English if you can use something else instead, although in this case I believe it is nothing more than the sound which sounds wrong.
In the former example you gave, the two '-ing' words have a different syllable count, while the latter example both are single-syllable words. This is a simple case of the former example sounding fine, whereas the latter example just sounds as if you are repeating yourself. Thus, it is considered bad use of English.
This isn't unique to 'ing' words or nouns, though. You may have noticed I used the words 'perfectly' and 'grammatically' together. It doesn't sound proper but is nonetheless legal. I simply couldn't escape using it in this instance.
EDIT: As alluded to by others here, the latter example, "I'm planning going on a diet" (which should actually be "I'm planning on going on a diet"), will almost without exception be phrased either "I'm going on a (new) diet" or "I'm going to go on a diet" (used when emphasising intent more than what you are intending to actually do).
I recall a terrific example of just how crazy this English language construction can get:
"This exceeding trifling wilting, considering ranting criticizing conerning adopting fitting wording being exhibiting transcending learning, was displaying, notwithstanding ridiculing, surpassing boasting swelling reasoning, respecting correcting erring writing, and touching detecting deceiving arguing during debating."
The author in 1851 is criticising this apparent non-violation of grammar. The quote translates to:
"This very superficial grammatist, supposing empty criticism about the adoption of proper phraseology to be a show of extraordinary erudition, was displaying, in spite of ridicule, a very boastful turgid argument concerning the correction of false syntax, and about the detection of false logic in debate."
So yes, two 'ing' words in a row is just fine and common practice!
