The ReplaceRepeated command performs replacements on a specified expression until the expression no longer changes.

$\mathrm{with}\left(\mathrm{MmaTranslator}\left[\mathrm{Mma}\right]\right)\:$

$\mathrm{ReplaceRepeated}\left(x+x^2\,\left[x=b\,x^2=x\right]\right)$

$2b$

$\mathrm{with}\left(\mathrm{MmaTranslator}\right)\:$

$\mathrm{FromMma}\left(\mathrm{`ReplaceRepeated[x\; +\; x^2,\; \{x\; ->\; b,\; x^2->x\}]`}\,\mathrm{evaluate}\right)$

$2b$