TeX does not allow the appearance of the same pattern twice. LuaTeX, I think, has just changed the error message: ! Conflicting pattern ignored. Now with LuaTeX it is possible to add patterns after the format generation. But one of the reasons why adding patterns in the document is to override format patterns, so I think that the right behavour for aa1bb1cc %in the format aa2bbcc %in the document is to get the maximum number at each point, therefore resulting in aa2bb1cc Furthermore, if the user writes somting like aa8bb %Do not brake here it will or will not work according as to the pattern aabb already exists in the format or does not, which is not certainly what a user writing this pattern would expect. Javier A.