Vertical double pistons are always a bit tricky. Here now is a solution that you can also build in a row up to 15 times in a row.

My goal was to build a simple double piston that is flat in the ceiling/floor and expandable.

Instructions:

Here's the one from the ground:

Brown=Input
Light blue=extend
Orange=Run in

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Here's the one from the ceiling:

Stone steps=Input
Pink=exit
Blue=retract

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Here are a few more pictures of how to go overboard with it:
The 15x4 gate:


The technique behind it (same as above):