When trying to use chemfig I get two errors.First there is a "two documentclass" error and second a "undefined control sequence \chemfig".What can be done about this? Uninstall/reinstall doesn't help.
I would guess that your main document is called
chemfig.tex which is a bad idea as
\chemfig.sty is a thin wrapper which inputs the plain tex macros
so since the current directory of the main document (not of the current file) is usually at the front of the search path, this just re-inputs your main document and you get an error about two