How did the duplicate Voyager make it as far as the real Voyager?

The Timeless Jump had to be the real Voyager since The Alternate Chakotay and Ensign Kim survives Over 15 Years after the Attempt to return to Federation Space which would be impossible for the Demon Class version of Voyager

tl;dr: The "Dark Frontiers" jump probably didn't happen yet, but of the two other jumps between "Course: Oblivion" and "Demon", the clones probably performed one.

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You asked the right question with the wrong jump: Between Demon and Course: Oblivion, three (or two, as C. Tomm states) jumps took place, to quote from another question:

In "Night", they find a wormhole that takes them to the other side of a 2,500 LY "void" in space (don't get me started) which should equate to at least another 2 years. In "Timeless", they use the slipstream drive and take another 10 years off their trip (somewhere between 9,000 and 10,000 LY based on previous statements). Finally, in "Dark Frontier", they use a Borg transwarp coil to go another 20,000 LY, which Janeway oddly estimates to be 15 years (odd since up to now 1,000 LY = roughly 1 year).

The original Voyager had a head start. How much more could the clones travel thanks to their enhanced warp engines? That could already account for everything. However, a much simpler explanation is that at least one of the jumps was performed by the clones, which can be supported by a quote from another post:

Some of the episodes that we saw earlier in the season, was it that crew? Or was it the real crew? _{^{Ken Biller - Cinefantastique, Vol. 31, No. 11, p. 50}}

That statement was of course deliberately vague and can be used to hand-wave away any inconsistency, but let's try the math:

Originally, Voyager was about 75,000 ly away which would take about 75 years, so they travel about 1,000 ly/year. Therefore the "Demon" planet is about 5 years (end of season 4) ≡ 5,000 ly plus Kes' Gift of 9,500 ly, so in total about 15,000 ly from the Caretaker Array and thus 60,000 ly from Earth.

Best enhanced warp drive speed would stem from no significant travel until "Course: Oblivion", which means 30,000 ly/y (since the remaining distance is claimed to take only two years on enhanced warp), though it is suggested that the clones also travelled about a year, which slightly reduces this to 29,500 ly/y. The worst speed estimate stems from granting both jumps to the clones, plus one full year of normal warp travel, which puts "Course: Oblivion"'s starting point about 11,500-32,500 ly closer to Earth (depending on whether the Stardate in Dark Frontier is a fluke rather than truth), i.e. something between 27,500-48,500 ly to Earth, so two years of travel mean at least 13,750 ly/y. So in total, the enhanced warp drive is roughly ten to thirty times as fast as regular warp, between 13,750-30,000 ly/y.

Assuming the events in "Course: Oblivion" didn't span more time than a month, that means the enhanced warp drive can only account for up to additional 2,500 ly of catching up with Voyager, and certainly not all the jumps and definitely not overtaking them to be just 22 ly apart after turning back. Therefore, some jumps must have happened to the clones. Let's play through the scenarios:

N | T | DF | EW | ΔV | ΔC   | Δ
--+---+----+----+----+------+------
V | V | V  | 30 | 32 | ≤4   | > 28,000
V | V |    | 30 | 13 | ≤4   | >  9,000
C | V |    | 28 | 10 | 3.5±2| >  4,500
V | C |    | 25 | 3.5|10.5±2| < -5,000 too far...
C | C |    | 24 | 1  | 13±2 | <-11,000 way too far...
C | C | V  | 24 | 21 | 13±2 | >  6,000
V | V | C  | 20 | 13 | 20±2 | < -5,000
C | C | C  | 14 |  5 | 32±1 | ~-25,000

N = Night jump (2,500 ly), T = Timeless (~9,500 ly), DF = Dark Frontier (20,000 ly)
performed by V = original Voyager, C = clones
EW = resulting enhanced warp speed in 1000 ly/y
ΔV = resulting distance of V from "Demon" planet in 1000 ly
ΔC = same for C, but note that ≤1 month @ EW is assumed
     and the ratio other Y:Demon is unknown, thus the ±
Δ = resulting ly between V and C, negative means the C overtook V in 1 ly

So in summary, the numbers won't add up unless the clones also jumped. Possible other explanations:

• The clones travelled longer on enhanced warp. Since their analysis leading them to the conclusion that they're "Demon" clones happens quite early and is dated back about one year, this seems unlikely.
• Knowing the stakes, safeguards of the enhanced warp drive were ignored when rushing back towards the Demon planet. That only works if the clones performed one jump other than "Night" and doesn't seem plausible.
• The clones did the "Timeless" jump, but circumnavigated the Void of "Night". I don't know if that's enough to account for the 5,000 ly discrepancy though, and in fact that cannot have happened within one year at non-enhanced warp. The messed-up slipstream exit might of course also be held responsible
• The former point with both Voyagers swapped, though that would leave the clones mostly behind instead of ahead.
• A(nother) clone-jump that happened off-screen.

That seems most likely. In fact, given the identical memories, it is likely the clones also tried the slipstream technology of "Timeless"¹⁾. That gives us a possibility

N | T | DF | EW | ΔV | ΔC   | Δ
--+---+----+----+----+------+------
V | B |    | 25 | 12 |10.5±2| -500 - +1,500⁺

N = Night jump (2,500 ly), T = Timeless (~9,500 ly), DF = Dark Frontier (20,000 ly)
performed by V = original Voyager, C = clones, B = Both⁺
EW = resulting enhanced warp speed in 1000 ly/y
ΔV = resulting distance of V from "Demon" planet in 1000 ly
ΔC = same for C, but note that ≤1 month @ EW is assumed
     and the ratio other Y:Demon is unknown, thus the ±
Δ = resulting ly between V and C, negative means the C overtook V in 1 ly

A repetition of the "Night" jump is impossible due to the Vortex' destruction (that the other Voyager went through before undetected is unlikely, since the Vortex was rather difficult to stumble upon), which leaves me wonder how the clones coped with travelling through the full Void unless they travelled around as mentioned before...

So it is highly likely that the "Dark Frontier" jump hasn't happened yet²⁾ and that the original Voyager performed the "Night" jump. Then either both Voyagers performed a similar "Timeless" jump or only the clones did, but took about 5,000 ly of a detour. That might be explained by the slipstream experiment deviating from the path due to the abort, and the slipstream travel might provide the clones' means to skip the Void, which otherwise would probably have eroded their sanity as well.

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¹⁾ Maybe even creating a Paradox of futures where the future-clones of Kim and Chakotay saved the clone Voyager, or the real one while their real counterparts saved the clones or ... timey-wimey anyone?

²⁾Unless the clones took a 5,000 ly detour around the Void and managed to fool the Borg into thinking Seven was the real one when the clones took the Transwarp Coil. But 5,000 ly in one year pre-enhanced warp isn't possible.

The episode Course: Oblivion takes place in stardate 52586.3:

"Captain's log, stardate 52586.3. We've had a lot to celebrate lately – Tom and B'Elanna's wedding, Ensign Harper's new baby, and the continued health of our enhanced warp drive, which has taken us within striking distance of home."

The episode Dark Frontier, where the Transwarp coil was acquired, takes place in 52619.2:

"Captain's Log, Stardate 52619.2. We got another 20,000 light years out of the transwarp coil before it gave out. I figure we're a good fifteen years closer to home."

So actually, the real Voyager travelled the extra 20,000 light years after the events in Course: Oblivion.

Interestingly though, both production and release order were first Dark Frontier and then Course: Oblivion.