If a package tries to export a symbol that's already defined, it will
report an error. You probably tried to use a function only to discover
that you'd forgotten to load its file. The failed attempt at using the
function caused its symbol to be interned. So now, when you try to
load the file, you get a conflict. Unfortunately, understanding and
correcting the code which caused the export problem doesn't make those
nasty error messages go away. That symbol is still interned where it
shouldn't be. Use unintern to remove the symbol from a package before
reloading the file. Also, when giving arguments to REQUIRE or package
functions, use strings or keywords, not symbols: (find-package "FOO"),
(find-package :foo). 

A sometimes useful technique is to rename (or delete) a package
that is "too messed up".  Then you can reload the relevant files
into a "clean" package.