Hi,
Open this file, select the 5 curves, then _Join.
You get 3 open curves. 2 open curves simply disappeared.
I would have expected to get a closed polycurve and 2 open curves…
joinBug.3dm (21.6 KB)
Rhino 8 SR3 on Windows
Version 8.3.23354.9001
Well Rhino doesn’t really know what order to join them in in this case. Rhino 7 does the same thing. There is an enhancement request filed for more intelligent joining:
https://mcneel.myjetbrains.com/youtrack/issue/RH-42838/Wish-Intelligent-Join
What’s odd is that Join creates two bad objects with the duplicate lines - they end up with stacked control points on one end. I don’t know why that happens, but it looks like that is the result of Rhino trying to join the duplicate lines to each other first. The joined results are the bad lines with stacked control points. This actually appears to happen with any duplicate line joined to itself, not just your example. Not sure what should happen in this case.
I filed a separate bug report for this:
https://mcneel.myjetbrains.com/youtrack/issue/RH-79310/Joining-two-duplicate-line-segments-creates-a-bad-object
From 6 years ago 
