In the Star Trek Voyager episode "Course: Oblivion" we learn that the duplicate Voyager crew that was created in the Star Trek Episode "Demon" created a Voyager ship and were exploring the galaxy before they began to break down on a molecular level.
Do we know if any of the episodes we watched between the episode "Demon" and the episode "Course: Oblivion" were actually the duplicate crew?
No, and yes. But mostly no.
As far as the episode itself is concerned, according to staff interviews the decision was taken to have the crew of the Demon-Class Voyager return specifically for that episode. It follows that none of the prior episodes were (intentionally) about the doppelganger crew
"Bryan Fuller came up with the idea, what if we followed the adventures of those people that we left on the Demon planet?" - Joe Menosky - Cinefantastique, Vol. 31, No. 11, p. 49
and
"In terms of how it got started [...] we sort of liked the idea of picking up the mimetic crew, and finding out whatever happened to them," - Nick Sagan - Star Trek Magazine issue 114, p. 3
That being said, the door was left open slightly.
"We didn't want to answer a lot of questions, like, how long has that ship been out there? Some of the episodes that we saw earlier in the season, was it that crew? Or was it the real crew? It's kind of intriguing to think about." Ken Biller - Cinefantastique, Vol. 31, No. 11, p. 50
Likely. At least, if you try to argument inconsistencies in writing away...
The discrepancy noted in "How did the duplicate Voyager make it as far as the real Voyager?" due to the jumps of ten thousands of light years, one explanation (apart from off-screen jumps) is that the ~10,000 ly jump in Timeless was performed by the clones, which would make that episode a likely candidate. It is also possible the clones did the Night jump instead, or even the Dark Frontier one, though the latter is unlikely.
For most of the other episodes between "Demon" and "Course: Oblivion", there are no indications for Clones, though Counterpoint raised one question that could also be explained by the Clones not being as telepathic as the originals...
I'd say we never saw the duplicates. Mainly due to the fact that Chakotay and Tuvok, when attempting to find when the unaffected materials came on board, state all of the major events the crew have encountered. And the only recognizable event was the Deamon planet event itself.
To be certain, you would have to watch the episode and pick out any references to past episodes.
Certainly none of the immediately prior episodes showed the duplicate crew as Tom Paris is still a lieutenant in Course: Oblivion, whereas the real Paris got demoted to Ensign nine episodes previously (or eight if you count Dark Frontier parts 1 & 2 as a single episode) in Thirty Days.
