Without additional "design", the machine (and the track on it) looks like the following: https://postimg.org/image/nfkgammej/

The machine itself works fine, but if you place blocks on the slime block portion of the machine and let it move along with it, the machine will sometimes cause block duplication:
https://postimg.org/image/d483hyup7/ (example)

This happens randomly and quite infrequently, but it isn't too rare. As far as I've tested, the machine in the track about the length of the first screenshot mentioned above is capable of causing at least one duplication around 4 round trips.

After extensive testing, this is what I've figured out in regards to the "phantom block":

• The block is physically interactive with the player (and presumably with other entities). The duped block will display the same physical properties.
• The block is possible to be broken with hand and/or with designated tools (such as pickaxes for stone), but will not yield item drops.
• The block can be pushed with pistons.
  • It also works with sticky pistons, but the block disappears upon retracting.
  • The block can be pushed indefinitely until when the piston attempts to push 13 blocks at
    the same time, at which the game seems to check for the 12 block limit count, and the
    phantom block is removed.
• The block does not recognize incoming redstone signals.
• The block is removed upon updating it. The following ways can effectively remove any at all of the duped blocks:
  • Placing a block next to it. This has to be performed in a way that you are directly facing the
    block.
  • Setting on fire with flint and steel.
  • Causing contact with water (and presumably also lava)
  • Right-clicking on it.
• Redstone components (such as redstone blocks, redstone lamps) seems to be duped this way, but the active machine will update the block and remove it after its next piston motion.
• Light sources (such as glowstone) seems to be duped this way, but the active machine will update the block and remove it after its next piston motion.
• Most phantom blocks will refuse to update properly. (Tested with randomTickSpeed=1000)
  • Phantom grass blocks will not cause other dirt to grow grass.
  • Phantom ice will not melt, even with the light source nearby.
  • Phantom magma blocks will not inflict damage to the player.
  • Phantom blocks affected by gravity (such as sand variants, gravel) or those which normally check for the floor (such as carpet variants, rail variants) will neither fall down nor disappear, but instead hover in mid-air.
    • Phantom rail variants in particular behave strangely in regards to duplication; The duped rail can and will be "caught" by the slime blocks of the machine. I attempted to exploit this by cloning a clone, but when the game "attempts" cloning, the clone will just be "slid off" of the slime block.
  • Exception: Phantom soul sand displays all physical properties normal soul sand displays, including extra-slowdown when an ice is placed below the block.
  • Exception: Phantom fence variants, cobblestone wall variants, iron bars and glass pane variants will still check for its surroundings and connect to other aforementioned blocks, phantom or not, just like normal blocks of those kinds do.

I'm sorry if this bug is a duplicate issue of something else, but I believe it is something worth checking out, if you haven't already.