According to some embodiments, a double detection apparatus for a mail-piece inserter includes a detectable flag affixed to a rotating mail-piece gripper of a mail-piece rotary feeder. A stationary proximity sensor may generate a voltage output based on a presence of the detectable flag in a direction along a first axis normal to the rotation of the mail-piece gripper. A decision unit may then generate a double detect alert signal when a phase shift above a pre-determined threshold value is detected in the voltage output generated by the stationary proximity sensor. According to some embodiments, the decision unit may also generate a thin detect alert signal when a phase shift past another pre-determined threshold value is detected in the voltage output generated by the stationary proximity sensor.