In any case SelDup will not consider a rebuilt curve the same as the original, as its geometric definition is different - SelDup does not look for something “close”, it looks for something “identical”. You can try this with a line, make a copy of it in place, rebuild the copy to degree 3 and 4 points, then try SelDup. It won’t select, but the rebuilt line is still for all intents and purposes the same line.
The point I was trying to make is that if you have a polycurve that you want to make into one single curve, Rebuild is one way to do this. You need to check the difference between the original and the rebuilt curve with CurveDeviation to see if it’s within acceptable tolerance. More points on Rebuild will generally get you closer, but the curve will be more noisy and harder to edit.
You can also try FitCrv and give it a fitting tolerance, make sure the AngleTolerance setting is very small though otherwise you may not eliminate all the kinks in the curve.