This is how skills are supposed to work!
If you are in a situation where there is only one person doing something, and they are rolling a single skill check, then yes, this is how it's supposed to work. Giving help is a natural thing and should be used in situations like this. There is no reason to prevent it unless the task is clearly something that's not going to benefit from someone else giving you assistance. There are some things you can do to limit it.
It's also worth noting that helping can often save you some table time. As a AceCalhoun points out in the comments, in many cases what happens if you don't help is that everyone in the party tries their hand at the task. This behaves very much like advantage, but with a slightly lower overall modifier (because most likely you'll have one character who is good at a task and the rest that are lower). So Working together only raises the change of success slightly and consumes less table time in these cases.
Be a bit more stringent about what you allow for assistance. Is coaching stealth really all that helpful? do you really want someone looking/talking over your shoulder while you're picking that lock? Evaluate situations where characters attempt to aid more carefully.
Have more than one thing going on at once. If all the characters need to be stealthy, they can't be helping each other. And if you need two arcane characters working on the sigils on opposite sides of the room, maybe they have to choose which one gets help from the third (or don't have anyone to help at all).
Make things take multiple rolls and limit helping on all of them. Maybe the first roll the wizard can be helped, but after that he's on the other side of the trap, or arm deep in the sigil or something to where additional assistance isn't going to help him.
Figure out how to inflict disadvantage for the task. Maybe there are mitigating circumstances.
Create a distraction. A rogue can't help the wizard if he's busy fighting baddies. Make some skill checks happen in an occupied room. Make completing the skill checks the win condition rather than defeating the enemies.
The basic crux of all of this is that helping is supposed to a mundane task that provides advantage. Yes, that's a huge deal, but it also doesn't stack with other things that give you advantage and it can be easily cancelled by disadvantage.
So get creative! Build some situations into your adventures that prevent your heroes from helping each other (or make the opportunity cost higher). But don't do it all the time, that might get tiresome. Adventurers like to help each other out, let them, but don't make it easy all the time.
Not exactly. The rules on Working Together in order to give advantage are as follows:
A character can only provide help if the task is one that
he or she could attempt alone. For example, trying to
open a lock requires proficiency with thieves’ tools, so a
character who lacks that proficiency can’t help another character in that task. Moreover, a character can help only
when two or more individuals working together would
actually be productive. Some tasks, such as threading a
needle, are no easier with help.
In other words, his familiar can only help him if the task is one that
a) The familiar is capable of accomplishing by itself, and
b) The task is one where the familiar's help would actually assist the character in completing the task.
It's up to you, of course, to decide if a given task satisfies these requirements. But you certainly shouldn't just let him gain advantage on every check automatically.
Best Answer
There is no flanking rule as a default; there are, however, a couple of optional rules proposed in the DM's handbook. It should also be noted that these are part of a general set of rules dedicated to playing using miniatures and tactical maps and even then they are marked as optional.
The flanking rule states:
An alternative optional rule offered by the DM's guide is called facing.
This rule suggests the following effects:
This is arguably more complex to adjudicate and manage than flanking is.
A potential issue one could take with flanking in 5e is related to the change in Attack of Opportunity rules. Previously, circling a creature tightly, even with a 5ft reach would provoke an AoO, as you were moving through threatened squares. In order to flank and avoid an AoO you would typically need to go wide or use an ability. In 5e an opportunity attack is only provoked when you move out of a creature's reach without disengaging. In my opinion this makes it effectively trivial to flank.
As a further counter argument, keep in mind that DND 5e adds the "help" action, which can easily be used as an analogous — yet more general and less powerful — mechanic.
In order to create an advantage, one attacker must focus on helping via a distraction or combat engagement at the cost of not being able to effectively score a hit. However, this still gives the other attacker a far better chance at scoring a hit. This makes this more useful against powerful opponents while being unimportant against lesser ones. I feel this makes this a much more tactical choice than mere positioning, while still providing a simple and generic mechanic that feels balanced and is unlikely to be overused.