Operating system: Linux-5.4.0-42-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce GTX 1070 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 440.100
Broken: version: 2.90.0 Beta, branch: master, commit date: 2020-08-25 16:00, hash: rB21cb6f09ffa8
Worked: (newest version of Blender that worked as expected)
Short description of error
When duplicating a bezier or nurbs curve and selecting the origin curve it is not highlighted as selected. or the duplicate is "overshadowing" the origin object. Since every duplicated mesh is highlighted this is not expected behaviour and probably a bug. you can select them alternatingly from the viewport and see the selection in the outliner, but not in the viewport.
I found this in 2.83.5 and tested in 2.9 too.
Exact steps for others to reproduce the error
add bezier curve. duplicate. select origin object.