Akai MPC User Manual page 292

Half Length: This will delete the second half of the Sequence.
Double Length: This will double the Sequence and copy all events from the first half to the second half.
Hardware: To edit the current Sequence:
1. Use the Cursor Buttons to select the Seq field.
2. Press Window to open the Edit Current Sequence window.
3. Press F3 (Double) to double the Sequence and copy all events from the first half to the second half.
The Edit Current Sequence window in the MPC hardware display.
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