Summary:
After placing an item frame on the side of an Atomic Reconstructor machine block and attempting to right click the item frame with a lens, the lens is instead placed on the machine block as if using the lens on that block, but lens is not removed from player inventory, thus duplicating the lens. This only works if the lens in question is in the player's offhand and their main hand is empty.

Recreation:

1. Place an Atomic Reconstructor Machine Block

2. Place a Vanilla Item Frame on the side of the Machine Block

3. Hold any Actually Additions Atomic Reconstructor Lens in player off-hand/shield slot and keep main hand empty.

4. Right click on item frame

5. Remove the lens that is now on the front of the machine

Player now has 2 of the original lens, and process may be repeated indefinitely

I've only had this glitch work when interacting with an item frame on one of the horizontal sides of the Machine Block, but not on the top or bottom.
This glitch was encountered within a 1.12.2 version of this mod within a modpack (Dungeons, Dragons, and Space Shuttles) so it is possible this glitch is a product of conflicting mods.

While this may be a very minor glitch, with some more expensive lenses this could be considered a rather serious issue within some modpacks or servers.