"once done so" is a grammatically correct and usable phrase in written English.
You can use it to refer to something that has already been done in the past. For example, "They had once done so much for me, I had to find a way to repay them."
"Someone who was struggling to make sense of what no longer made sense, and to understand why it had once done so.
China had once done so, but since about 2014, it has basically been engaged in the reverse practice: propping up its currency to stop capital flight.
"If it was once done so perfectly, why do it again?" Mr. Greene once asked aloud, voicing perhaps the most obvious question about his devotion to Jelly Roll Morton.
If there is a substantial population that no longer affiliates with any religious tradition, however faintly they may have once done so, it is necessary to understand where their ethical moorings extend.
Like Facebook once upon a time, all users are required to log in with their.edu email addresses to prove that they are either currently attending a participating university or had once done so.
Furthermore, unlike Watson, Zeman's patient (in common, indeed, with the patients in the other cases of neurological loss of conscious imagery, just mentioned) was in no way inclined to deny that other people experience visual mental imagery, or that he himself had once done so.
Black cricketers who played against the West Indies once did so expecting to take a deal of stick.
Just as the term "broadcasting" no longer covers what it once did, so "television" has become ambiguous.
Several of Britain's ex-colonies once did so but only because English was a neutral language among competing native tongues.
Among the most peculiar is USEC, previously known as the United States Enrichment Corporation, which enriches uranium for reactors and once did so for bombs.
More than anything else, sport reminds you of how your body is ageing and how you are no longer able to do what you once did so freely.
