We can check how much we get in return when we equip multiple gear with the same stats by trying them out one at a time, but thanks to a redditor u/DesuSnow for the post, we can see the rate of return in percentages.
"1st" simply means the equipment that you have with the highest stat, 2nd is the second highest that you have, and 3rd for the one with the least amount.
Giving a character 3 pieces of equipment that gives HP like +600, +300, and +200 means that you get +600 + (300x50%) + (200x30%) = +810 HP. This is assuming that you have max proficiency for all the gears to get the full benefit of the stats.
From the image shown above, we can see that some stats like DEF and SPR have very low rate of return from additional pieces, making it not very effective to stack them at all.