Merhabalar! I am used to thinking that "istemek" takes the infinitive when it is working with another verb. For instance:

Ben okula gitmek istiyorum.

I am studying at Duolingo, and I was given this sentence to translate into Turkish:

I really want to create again.

Of course, I tried to use the infinitive for "create." But here is Duolingo´s correct answer:

Ben tekrar yaratmayı gerçekten istiyorum.

What is changing the usual rule here? Is it just the insertion of "gerçekten"? Or is something else at work?

Hope I am thinking correctly about all this. ... Teşekkürler!

I am a native speaker and I don´t know how to answer such question

To me, there is no difference between -mek istiyorum and -meyi istiyorum in any context. Tekrar yaratmayı istiyorum, tekrar yaratmak istiyorum they are both fine and mean the same thing. I use & hear the second form more often.

By the way, I want to add something.

You do not need to use " Ben" in the beginning of sentence, actually you can simply say "yaratmak istiyorum" or "yaratmayı istiyorum"

Using "Ben" in the beginning of sentence is optional.

For me, "yaratmak istiyorum" sounds better than " ben yaratmak istiyorum",  and native speakers generally do not use "ben".

Actually, it´s a matter of definitude.

Both of the translations are correct, but if you want to express your idea or opinion with more definitude you better use with -meyi/-mayı in turkish. but the usage of both expressions is directly about the question. if the question is more common then the answer with infinitive would be better (still the other usage is also correct).

if the question is more detailed then the answer with -meyi/-mayı would be better (still the other usage is also correct).

ghhhhh

examples:

q.1: What do you want? (ne istiyorsun?) [explanation: as you see, a very common and simple question; just "what do you want?"]

a.1: I want to go to school (okula gitmek istiyorum) [exp.: by the way, "okula gitmeyi istiyorum." is still correct and formal]

q.2: What else do you want? (daha ne istiyorsun?) [exp.: you wanted blaa, blaa, blaa... "what else do you want?", as you see it´s a bit more complicated]

a.2: I really want to create again (gerçekten (de) yeniden yaratmayı istiyorum.)) [exp.: by the way, "(gerçekten (de) yeniden yaratmak istiyorum.." is still correct and formal]

As a conclusion, the difference is quite slight. that´s why both of the translations in turkish are correct and formal.

the point here is the detail of the question. 

Yes to me it seems that the position of "gerçekten" is a factor here.

Ben tekrar yaratmak gerçekten istiyorum. (sounds a little bir wierd but if we change the order...)

Ben gerçekten tekrar yaratmak istiyorum. (sounds OK)

My conclusion is if you want to use an adverb (like gerçekten) it should better come before infinitive:

aslında gitmek istemedim.

iyi başlamak istiyorum.

kötü bitirmek istemiyorum.

Well, as a student of Turkish, I guess it´s encouraging to know that even native speakers can find a point like this murky. On the other hand, we students like concrete rules. I guess that´s what makes language difficult -- sometimes there are no concrete rules. ... Açıklamalarınız için çok teşekkürler!