It seems that the cave length calculation count shot AND back-sight shots / or duplicates. These should be excluded from the length calculation.

Just tested again. As long as the direct shot and backsight shot are not merged, the cave length is tawice the length obtained after merging. Duplicates should be identified and not counted in the cave length even if they are not (yet) merged.
You can test with the file I sent you (FP2.csv).