Bug(?): Re-definition of alternative sprites is not treated as an error but has no productive use

NML warns when alternative_sprites with the same zoom level and bit depth are defined twice, and overwrites the earlier definition with the later definition. However, there is never(?) a productive use of this behaviour, as alternative sprites are effectively part of the real sprite, so can't be redefined without redefining the entire real sprite.

replace grid_temperate_spr3981(3981, "temperate_groundtiles_gridline_8bpp.png") { template_groundtiles(0, 0, 1) }
alternative_sprites (grid_temperate_spr3981, ZOOM_LEVEL_NORMAL, BIT_DEPTH_32BPP, "temperate_groundtiles_gridline_bt32bpp.png") { template_groundtiles(0, 0, 1) }
alternative_sprites (grid_temperate_spr3981, ZOOM_LEVEL_NORMAL, BIT_DEPTH_32BPP, "arctic_groundtiles_gridline_bt32bpp.png") { template_groundtiles(0, 0, 1) }

Gives a warning like:

nmlc warning: "test.nml", line 47: Sprites are already defined for replace-block 'grid_temperate_spr3981' for this zoom level / bit depth combination. This data will be overridden.

This is reported as a warning at the same level as the 8bpp sprites containing white or animated pixels. However, as there are no(?) valid uses for redefining an alternative sprite set then it should probably be an error.

Reproduce the duplicate alternative_sprites definitions in test.nml, using the same zoom level and bit depth shown in the report. Start from the alternative_sprites handling and its existing warning, then verify that this duplicate case produces an error rather than an override warning.