This Week At Bungie tends to provide developers' insights and future plans for Destiny 2 and this time around they talked about Armor 2.0 and how it's going to evolve in the future. One of the planned changes is adding diminishing returns to mod stacking.
Destiny 2 Armor 2.0 changes that came with Shadowkeep eased the life of many players who weren't keen on mods being consumable. Bungie recognised the newly gained freedom players found while experimenting but some restrictions were still in place.
Therefore, one of the future changes revolves around removing restrictions that don't let players stack the same mods on the same piece of armour. For example, players will be able to use two Hand Cannon Loader mods, which is not currently available. Bungie predicted that stacking mods can easily become overpowered so they put some measures in place.
Mods will start yielding fewer benefits as you keep stacking the same ones on the same armour slot. In the example of the Hand Cannon Loader mod, using two of them will not double the reloading speed as one mod will provide only half the benefit, for a total of 150 per cent of the benefit a single mod would provide.
In other words, adding the second mod only gives half the benefit that the first one provided. While this may sound a bit brutal, it will provide newer Guardians with means of combatting the enemies that already slotted an Enhanced version of the mod.
While diminishing returns sound bad on paper, it's important to realise that Bungie are actually giving players more options here and this is, therefore, a positive change . It will kick in with the next season.