SugarSync also does just that: the file is duplicated and the user is left to resolve the conflict at a later time. Actually it does that in a really clever way

SugarSync also does just that: the file is duplicated and the user is left to resolve the conflict at a later time. Actually it does that in a really clever way so that if users keep editing over the already conflicting versions, it doesn't create new conflict versions but rather treats the new files as further versions of existing conflict duplicates...