Walls: Shot data with duplicate "from" and "to" stations is handled differently by cavern vs walls

[Joe Walko]

When shot data with duplicate "from" and "to" stations is encountered, Walls creates two distinct shot vectors, and uses "loop closure" logic to calculate the coordinates.

Survex, on the other hand appears to simply average the values of the duplicate shot data, and creates a single shot vector.

Given the Walls SRV data below: Walls length for a-b is 15. Survex length for a-b is 20.

#FIX a 0 0 0
a b 10 90 0
a b 30 90 0

Walls shapefile vector export shows two distinct, but identical, a-b shot vectors, each 15m in length. Survex, otoh, indicates a 20m single shot vector in log and .3d data.

Also: It may be prudent for cavern to log a 'warning' for duplicate stations. Walls has this capability, but it's disabled by default, and most users don't bother to enable it (Options->Compilation->Look for duplicates).