Learn Dutch: Hij bestelde toevallig hetzelfde. - He coincidentally ordered the same thing.

Because we are talking about an unspecified concept or "thing" in general. When "the same" stands alone as an independent pronoun meaning "the same thing", Dutch always uses the neuter form hetzelfde. If it referred back to a specific "de" word previously mentioned, like de pizza, you would use dezelfde.

Is hetzelfde always written as one single word?

Yes. Unlike English where it is two words (the same), Dutch always combines them into one word: hetzelfde or dezelfde. This is true whether it stands alone, like in this sentence, or is placed directly before a noun.

What does the adverb toevallig add to this sentence?

It translates to "coincidentally" or "by chance". It is a very common word in spoken Dutch. You will often hear it used like this to describe a pure coincidence, or in questions to mean "happen to", such as asking if someone happens to know the time.

Why does toevallig come before hetzelfde?

In a standard Dutch main clause, adverbs like toevallig usually come immediately after the conjugated verb (bestelde) and before the direct object (hetzelfde).