When running glxinfo, or using my own code (calling glXChooseFBConfig to get a list of GLX framebuffer configurations), I see that there are entries which are identical except for their ID code.
For example:
$ glxinfo
...
0x77 0 tc 0 32 0 r y . 8 8 8 8 4 24 8 16 16 16 16 0 0 None
...
0xae 0 tc 0 32 0 r y . 8 8 8 8 4 24 8 16 16 16 16 0 0 None
...
What is the reason for this duplication? Is there an underlying difference between these seemingly identical modes?
Though they have the same buffer configuration from the point of view of OpenGL, they differ from the point of view of X11. Specifically, they have different X Visual bit-depths (one is a 24-bit X visual and one is a 32-bit X visual)
[note: I found this out while I was composing the question, but since I hadn’t found an answer in my web searches I’m posting the question and answer here anyway — maybe someone else will find it useful in the future.]