The short answer
A fake Duomètre moon phase is a replica of the Jaeger-LeCoultre Duomètre à Quantième Lunaire, and in nearly every case it copies the look, not the mechanism. The moon disc turns, the window sits where it should, but the gear underneath is a plain 29.5-day wheel driven off the going train. If you typed "duomeetre", you mean Duomètre, JLC's dual-wing line. Knowing what the genuine watch does makes the replica much easier to judge.
What the genuine moon phase does
The Duomètre à Quantième Lunaire runs on the Dual-Wing idea: two separate gear trains fed by two barrels, one for timekeeping and one for the complications. Running the date, the moon phase and the jumping seconds does not steal energy from the balance. JLC rates the moon phase so it needs a single correction about every 122 years. A true lunar month averages 29.53059 days, which is why a properly geared moon disc stays close to the sky for a lifetime.
Why that number matters to a buyer
Most replica moon phases use a 29.5-day wheel. The gap per lunation is 0.03059 days, and with about 12.4 lunations a year that adds up to roughly 0.38 days annually. In plain terms, a decent replica moon disc drifts a full day off the real moon in about two and a half years. That is fine for a watch you wear on weekends. It is not fine if a seller claims lunar accuracy.
Where the replicas usually fall down
- The disc itself. Genuine moon discs are thin metal, often grained or lacquered, with a moon graphic that sits under the aperture with depth. Replicas print on flat plastic or foil, so the moon reads like a sticker under glass.
- The star field. Look at the sky around the moon. On cheap copies the stars are a repeated dot pattern, sometimes mirrored onto the other half of the disc. The real field has variation.
- The jump. Many fakes move the disc continuously, so at any hour the moon sits between positions. Genuine discs advance in one step per day.
- Aperture shape. The window should be crisp, with a bevel and no burrs. Sloppy stamping shows a wavy edge or uneven corners.
- The seconds hand. The Duomètre flying seconds hand ticks in six steps per second. On replicas it sweeps like a normal automatic, or sits frozen at six o'clock because the module is decorative.
- Dial furniture. Subdial rings, printing weight and the date window look off at 2x magnification. The font is a common giveaway.
Five checks before you pay
- Watch the moon for 24 hours. Photograph it, wait, photograph again. It should move one step, not glide.
- Track a full cycle if you can. A 29.5-day wheel returns to the same position on day 30. A 29.53-day wheel does not.
- Shoot the moon graphic with a phone macro lens. Printed dots and flat color mean a low grade disc.
- Set the time near midnight and watch date and moon together. Both should snap, not crawl.
- Ask what movement is inside. If the seller cannot name a base calibre, assume a generic automatic with a decorative plate on top.
Price bands and what you get
Under about $150 you get quartz or a basic automatic with a moon window that changes on a fixed daily cycle no matter what the sky is doing. Between $200 and $500, working 29.5-day moon phases appear and the case finish improves. Above $600 the better factories spend money on the dial, the moon disc and the case profile. None of them reproduce the Dual-Wing architecture, because that needs two gear trains and a level of finishing no replica line bothers with. Anyone advertising a "true Dual-Wing clone" is stretching the truth.
Bottom line
A fake Duomètre moon phase can be a good-looking watch and a fair way to enjoy the design. Treat the moon phase as a moving picture, not an astronomical instrument. Check the step, check the disc, and buy from a seller who names the movement instead of hiding it.