t=11，n=280，不知道哪一步出了错？
结果如下：
spgmmxt ln_tax_gdp lnpgdp ln_eximport ln_urban ln_pop_den ln_openness ln_fixass_gdp ln_fisexp_gdp ln_firgdpratio city_id , nc(280) wmfile(W_eco) gmm(1) mfx(lin) test

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*** Binary (0/1) Weight Matrix: 3080x3080 - NC=280 NT=11 (Non Normalized)
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* Spatial Panel Autoregressive Generalized Method of Moments (SPGMM)
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new variables cannot be uniquely named or already defined