economists classify production as possessing constant decreasing or increasing

economists classify production as possessing constant decreasing or increasing returns to scale. Yet from a cause-and-effect point of view it is not readily
apparent why decreasing returns to scale should ever exist. That is if we duplicte an activity we ought to get duplicate results. Hence if we truly duplicate
all of the inputs we ought to get double output. Can you reconcile the apparent contraction between this logic and the expectation of the economist that
beyond certain output ranges firms will confront decreasing returns to scale