The spec in stop_times.shape_distance_traveled mentions (link):

Values used for shape_dist_traveled must increase along with stop_sequence; they must not be used to show reverse travel along a route.

Currently, the validator treats both decreasing values and equal values of shape_distance_traveled as an error.
We should differentiate decreasing values (error) and equal values (warning).
The same discussion happened for the rule DecreasingOrEqualShapeDistanceNotice, which was separated into different rules.

Additional context in this comment.