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Cyclone_Joker
Sidestepping the issue of whether Generalist and Domain both replace the same ability (which seems to have quite a few other people out there on the interwebs who think that they aren't doing that on a slim technicality), how about this:
A Domain Wizard cannot take Generalist Wizardry because by RAW a "Domain Wizard" is not a "Wizard" (see the excerpt below from UA about the Bard and Bardic Sage being merely "very similar" classes). An additional argument/reinforcement is given by the Generalist Wizard only being available to a "standard wizard" by the RAW of RotW. By UA RAW, variants are not "standard class".
From UA
"Each fully detailed variant has entries for one or more of the following topics. If an entry does not appear, use the material for the
standard class."
"Wizard Variant: Domain Wizard"
"Under no circumstances does spellcasting ability from
multiple classes (even variants of the same class) stack. A character with levels of
bard and levels of bardic sage has two separate caster levels and two separate sets of spells per day,
even though the classes are very similar."
From RotW
"A substitution level is a level of a given class that you take instead of the level described for the
standard class."
"To take an elf wizard substitution level, a character must be an elf about to take her 1st, 3rd, or 5th level of
wizard."
"This substitution feature replaces the
standard wizard's ability to specialize in a school of magic."