Repeat Calibration Level

Find the first pair of calibration checkpoints that end at the same cumulative level.

A device is recalibrated by applying adjustments in order. The device starts at level 0, and each integer change raises or lowers its level. After each change, record the resulting level. Return a two-item list [earlier_checkpoint, later_checkpoint] for the first checkpoint that reaches a level already reached at an earlier checkpoint. Checkpoint 0 is the starting level before any changes, and checkpoint 1 is after the first change. If no level is reached twice, return [-1, -1].