I just found out that 2 same boss jewels stack and give double stats.
I just tested this and they don' t. Perhaps you could show me how you did it with screenshots? Also, let me point out that if you have 2 identical jewels and remove 1, bonus will fade away. Consequently, somebody could be deceived that second jewel had given bonus after all. Please make sure it' s not what i explained above, as well.