When items are inserted into the top of the Rolling Machine they are duplicated and become a full stack. I discovered this issue from Thermal Expansion machines, and tested it with items coming straight through a transposer, transposer to tube, and a wood buildcraft pipe. All instances except the buildcraft pipe caused duplication. To ensure that the machine wasn't using an adjacent inventory I put one diamond in the Rolling Machine to be duplicated and one in the chest against the transposer, and when the diamond made its way to the Rolling Machine it filled the slot with diamonds.

EDIT: I forgot to mention I'm using Railcraft 6.12.1.0

Should be fixed in next version.