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This tune is from R2 episode 23, near 19:20 mark in the version I have, though it is played many times in the series.
The tune plays at 7:15 in this video.
Does anyone know which musical piece this is?
It's an unreleased piece from the soundtrack called Path of Decisions. You can find it here:
Code Geass: Unreleased Album - 30 Path of Decisions