The current calculation is well covered by other articles, but I think there are some additional factors to keep in mind. From the point of view of balance, these factors may make the proposed rule more consistent with your objectives. This is not meant to be as critical as simply reporting cases where it makes a substantial difference.
What is your main goal
Your statement that "this way players can stack +1 for each smart action" seems to indicate that you may be focused on situations in which the DM gives a discretionary benefit to people for a smart plan, a good game of role, etc. depends entirely on the DM, and with a sufficiently generous DM can make it a very advantageous approach in many situations. Essentially, it would be to name a modifier to the DM based on the number of ways he thinks the player was smart in his plan or not. It seems to be more work for the deputy minister than simply saying "you have a benefit" if there is something worthy of a benefit, but it would give the deputy minister more flexibility in determining that. who is right. Of course, they can already edit the CD to take into account the factors that players can take into account in their approach to the task. This is therefore of little importance if the DM is comfortable with making such adjustments.
If you are a DM for whom the main purpose of this internal rule is to give people intelligent approaches to competency control, I strongly recommend that you simply adjust the CDs according to their approach, and not perhaps not even give any advantage in these situations if you do not do it. I do not like double rolls. There would then be no problem balancing beyond that.
A multiple advantage would be more or less abundant and important in different types of circumstances
There are many situations in which a constant advantage is dictated by the rules, namely various class and exploit characteristics, various spells, objects, and help action.
Since the form of benefit you offer is lower in most circumstances and the characteristics of the class (barbarian reckless attack for example) and the characteristics of the exploit (master of the shield for example ) are mainly major mechanical aspects of the class, subclass, exploit, etc., this can radically weaken these character building options. These are primarily combat-focused sources of advantage, and opportunities to stack a combat advantage will generally be less common as there are simply too many other uses for people's actions, bonus actions, and so on. in combat. Perhaps the barbarian suffers the most on this front, as reckless attack is a major feature of the class and brutal criticism is also a major feature that is twice as easy to trigger when it's in the dark. a benefit is granted (critics are explained below).
Spells, such as fairy fire, are one of the other major sources of automatic automatic advantage (and disadvantage), especially in combat. This radically changes the value of these spells. Especially for low-level launchers for whom spell casting is one of their most powerful contributions to combat, this is a substantial change, and this also applies to disadvantages. The bard is especially thought of, as her main injury damage is a vicious mockery, low in damage but causing a disadvantage, and for whom direct damage spells are quite rare until she manages to loot the Spell lists from other classes at higher levels (not up to level ten for non-Lore Bards).
Some items, such as Elvenkind Boots, also confer a benefit on something. You have to ask yourself whether you really want to do them or not an object on some kind of role.
Finally, we have help action. Being able to grant this as a bonus is one of the main features of the Mastermind Rogue (I would even say it's the only really powerful feature they have in the first two levels), so we have another class that is now well balanced . differently. Being able to tune through a pet is also a major aspect of the wizard.
Beyond that, most players rarely act, but few party members spend money fighting for help. It is therefore unlikely that such heavy benefits will accrue. In the meantime, in the case of many abilities checks, there is no reason why the whole party does not help a player to roll if the DM allows it. This will depend to a large extent on the ability of the DM to let several people try the same skill test and let the players use the help in such circumstances.
There is also the aspect of the help action that often involves one player dropping his throw so that another player gets two throws. Whatever the mechanical forces, it makes it a simpler choice for the player and a source of camaraderie for some players.
The natives and the twenties
If you only roll a dice in a fight, chances are half as likely, which is another reason for it to be much less powerful. It is particularly important to keep this in mind when it comes to the balance of classes or subclasses for which a substantial advantage is related to criticism, such as a barbarian fighter or a champion. A critical failure would be more likely because you have half the chances of avoiding one, but they are much less important because they are usually part of a series of numbers that will obviously not be affected, but this is also related to a few. mechanisms such as the halfling "luck" and would have an effect of balance.
Also consider that, on many tables, the principle that a natural factor of twenty-one leads to a dramatic critical failure or a success in the ability controls independently of the modifiers (it is probably the rule the most common of the house). For the benefit, it halves the chances of critical success and doubles the chances of critical failure.
The application of the rule comparable to a disadvantage, if you adopted it also, would of course have the opposite effect for all this.
Advantage and disadvantage?
An advantage and disadvantage aspect do not Stacking means canceling yourself completely, no matter how many sources you have on one side or the other. You must also consider how you will handle this rule.
I assume you wanted this rule to apply to all benefits rather than your added benefit. If this is not the case, the meaning of this rule is very different and much weaker for the very reasons that have been explained in this answer and in several others. It would simply be a modest (usually) buff (or if you apply it as disadvantageous disadvantage) to throws, and, unless you let the whole party take some steps to help with a skill test out of combat, it does not seem like it can really break any aspect of the game.