Inconsistent geometry orientation and conversion

I've run into an odd issue working with some data found in the wild. Consider this MRE:

library(sf)
library(s2)

# sf polygon as loaded from a database
poly <- structure(list(list(structure(c(12.9718058342186, 12.9718058342186, 
12.9728528296958, 12.9994465148162, 13.0258308008412, 13.0090788732063, 
12.9931645419531, 12.9676178523098, 12.9529599156293, 12.9223876476955, 
12.8830206177534, 12.8428159914296, 12.7963293922426, 12.7460736093378, 
12.7305012888338, 12.7291122826075, 12.7257618970805, 12.725552497985, 
12.7259712961759, 12.7351848563751, 12.7542401740599, 12.7718296980765, 
12.7860688365662, 12.785931374378, 12.801773768724, 12.8160129072137, 
12.8537047443923, 12.9047981236788, 12.9491907319114, 12.9718058342186, 
-4.04623539727417, -4.04623539727417, -4.05062180563782, -4.0750599304832, 
-4.1013770804975, -4.10806066333969, -4.12226309084477, -4.14356625610239, 
-4.16236269020371, -4.18742390394239, -4.21331964830488, -4.22626719749194, 
-4.23169675058643, -4.2296084654374, -4.23006522722775, -4.21833162843474, 
-4.18136751731289, -4.15421762373774, -4.1429397005941, -4.1201745144563, 
-4.09281365829253, -4.07861070745388, -4.06858494389262, -4.06833641784592, 
-4.05960342472729, -4.05166618507748, -4.04289335590868, -4.04080457305779, 
-4.0433111118331, -4.04623539727417), dim = c(30L, 2L)))), class = c("XY", 
"MULTIPOLYGON", "sfg"))

# the polygon contains degenerate vertices, so we fix it using s2_union()
as_s2_geography(poly) # Error:  Loop 0 is not valid: Edge 0 is degenerate (duplicate vertex)
as_s2_geography(poly, check = FALSE) |> s2_union() |> s2_is_valid() # TRUE

# now it gets weird... surely this tiny area does cover the entire Earth globe?
as_s2_geography(poly, check = FALSE) |> s2_union() |> s2_area() # ~ 510e6 km2
as_s2_geography(poly, check = FALSE, oriented = FALSE) |> s2_union() |> s2_area() # ~ 510e6 km2
as_s2_geography(poly, check = FALSE, oriented = FALSE, planar = TRUE) |> s2_union() |> s2_area() # ~ 510e6 km2

# doing a roundtrip conversion to sf fixes the area calculation
as_s2_geography(poly, check = FALSE) |> s2_union() |> st_as_sf() |> as_s2_geography() |> s2_area() # ~ 488 km2

# lwgeom also produces the correct calculation
lwgeom::st_geod_area(st_sfc(poly, crs = "WGS84")) # ~ 488 km2

It seems that s2 interprets the polygon as a hole on the Earth’s surface, which kind of makes sense due to the CW orientation of the vertices in the data. There are a few things that confuse me, however:

• Shouldn't oriented = FALSE take care of this by reorienting the polygon so that the exterior area is larger than the interior area?
• Why does the orientation change when converting it back to sf?

In general, how should one deal with situations like this? And how does one catch it to ensure correct data treatment? Right now, what I am doing is manually checking the polygon orientation and reversing the vertex sequence if needed (with logging to double-check the data later), which seems cumbersome?

P.S. If I manually remove the duplicate vertex at index 2, the result is as expected:

xy <- st_coordinates(poly) |> as_tibble()
xy <- xy[-2, ] # remove the 2 vertex
pp <- s2_make_polygon(xy$X, xy$Y, oriented = FALSE)

s2_area(pp) # ~ 488 km2

Start with the supplied MRE and trace the entry points as_s2_geography(), s2_union(), s2_area(), and st_as_sf(), comparing behavior with and without the duplicate vertex. Check the orientation and area results for each conversion path; done means identifying the cause of the inconsistent result and establishing the expected handling for this input.