Items duplicate spell effects, or cumulative?

[Gateway103]

It is to my understanding that 2 identical items will not have double the effect, with the exception of gem producing items like clams.

For example, having 2 rings of wizardy will not give you +2 to all existing path, or having 2 Endless Bag of Wine will not give +50 supply. Same with 2 Thistle Mace. The fact that 2 Brazen Vessel gives 2 blood magic may actually be a bug, unless the developer decides that blood magic needed the boost?

-Gateway103