When life forms of opposite chiralities exist in the same place, they compete against each other s by turning precursor hydrocarbons into forms that their mirror can't use. There is also the fact that while polymerization of alternating left and right handed nucleotides is relatively rare, it happens in molecules like siderophores.

Btw. Mirror chiralities are not exactly symmetric. There are tiny asymmetries between chirality due to weak interactions and electron interactions. Weak interactions might affect long hydrocarbon chains. Electrons from beta decay interact with molecules of different centrality with slightly different speeds (difference is something like 0.01%).