I imagine it is the obvious fact that the items give the same bonus. But does it boost that specific bonus or the armor pieces in general in any way?
For example I am wearing a Legendary Imperial Seal and Legendary Dark Rosary and I am not seeing any visible change in their stats when the label is there versus not.
Best Answer
The Same Bonus In Use applies to equipment with the same rarity, so you would see it often with all the legendary items you get at rarity 50. This applies a small boost to the stats of those items (not sure how small), and would be impossible to tell without doing the math yourself.
Personally i'm too lazy to add it up myself, but the previous games did the same thing (there may be some exceptions, but i started with D3). Like in Disgaea 4, if you wanted to max your stats, not only would you need R40 equipment, you would also want them to be the same rarity (I.E. Legendary Rarity 10 on all items) for that bonus as well, as well as a few other things to max out stats.
Keep in mind, I've checked some of those items and the higher the rarity, the better the stats are on the item in Disgaea 5. Also it seems a lot of legendary items are rarity 50.
Hope this helps, if i missed anything i'll edit my post.