ut_hb210_s2 cannot be passed as reform= (VariableNameConflictError) #9844

ut_hb210_s2 cannot be passed as reform=:

from policyengine_us.reforms.states.ut.ut_hb210_s2 import ut_hb210_s2
from policyengine_us.system import CountryTaxBenefitSystem

CountryTaxBenefitSystem(reform=ut_hb210_s2)

VariableNameConflictError: You've already defined ut_hb210_s2_marriage_credit in None. You tried to define it again in None.

This was run on the head of #9628 (743b6dfd3d, which is main at ea79c39fb7 plus that PR).

Cause

CountryTaxBenefitSystem.__init__ applies the reform it is given twice:

1. it applies it in if reform:, before structural-reform detection;
2. it applies it again, together with the structural reforms, after the modified flags are reset.

main does the same. ut_hb210_s2's apply calls self.add_variable(ut_hb210_s2_marriage_credit) (policyengine_us/reforms/states/ut/ut_hb210_s2.py), so the second application finds the variable already defined. Simulation(reform=...) builds the system the same way, through spm.py.

One fix is to skip the add_variable when the variable already exists. The test in #9628 guards a parameter it adds in the same way. Found while reviewing #9628, which does not change this.