 Statistics and symmetry factors of a two-propagator loop

A closed Dirac loop gives a minus sign and a spinor trace. Replacing it by two distinguishable real scalar species removes both, with no extra interchange factor. One identical real scalar species instead has Feynman-diagram symmetry factor two. A cubic interaction $g\varphi {\chi }^2/2$ has vertex $ig$, whereas $g\varphi {\chi }^2$ has vertex $2ig$; those choices give different numerical coefficients even though their drawn loop topology is the same.