A homogenous list is guaranteed to only have one instance unless you enable incoherent instances (a known bad extension) or confuse the compiler with orphan ins

A homogenous list is guaranteed to only have one instance unless you enable incoherent instances (a known bad extension) or confuse the compiler with orphan instances (though I've never encountered this and think it should be detectable at link time anyway). One type -> at most one instance for any class, in Haskell.