We first assume that there are already some people in your database and that you are adding people not connected to the people that are already there.
Go to Family View. (Descendant View can also work well, but I will describe the data entry process when in Family View). It doesn't really matter which person or family who is already in the database is displayed. Add Individual from the menu bar and enter at least the basic information for the new person such as their name and birth date. It's probably best to add one of the parents as the first person to be added. At this point your newly added person will be the only person displayed, and they will be in child's position in Family View. In some respects, this is unfortunate because it might be nice to have them in the parents position in Family View, but that's the way it works.
Right click the person and click Add Spouse... Go through the process of entering at least the spouse's basic information such as name and birth date. You can enter the marriage information now or later, but you really want to get the spouse into the system. At this point, the two new people are the only people being displayed - one in husband's position and one in wife's position. The "Father's Parents:" position will be blank. The "Mother's Parents:" position will be blank. The "Children" position will be blank. The important thing is that you now have the basic backbone of the family structure in place.
With the basic backbone of the family structure in place, you can now click on "Click to add father" for either the husband or wife, you can click on "Click to add mother" for either husband or wife, and you can click on "Click to add a child" to add add children. It should be pretty obvious where to click, and you should be able to avoid duplicate people without undue difficulty. You continue clicking "Click to add" to keep adding family members, and it should be pretty obvious where to click.
It is perhaps a little more difficult if the new people you are adding are in fact connected to the people that are already there. You would add a spouse as described above by right clicking the person to whom you are adding a spouse and clicking Add Spouse... The tricky part might be if the person to whom you were adding the spouse already had the same spouse entered and if the person to whom you were adding the spouse were in the child's position. Then, it might be easy to add the same spouse a second time, and this is the sort of thing you are trying to avoid. So before you do anything, highlight the person to whom you are adding a spouse and look at their number of spouses value in the upper left hand corner of the screen. It says Spouses:. If the number is greater than zero, then move the person to whom you are adding a spouse into the father or mother position instead of being in the child position.
This is an area where RM's user interface does not excel, in my opinion, and it can be a little confusing. The best way to move the person is probably to highlight the person who is in the child position,to switch briefly to any other view, and immediately to switch back to Family View. This will place the person in the father or mother position. At this point you can easily see the spouse and determine if the spouse is already in the system. You will also be in a position as described above for entering father's father, father's mother, mother's father, mother's father, and children.
Even though I think it's probably simpler for a new RM user to do these activities in Family View, there are certainly advantages to doing them in Descendant View instead and I suspect that many experienced RM users use Descendant View for entering new people rather than Family View. It would be good to play with both and learn how to use both. But the real key is to learn how to get the highlighted person into the correct position in the View so that you can so the other people around the highlighted person - the highlighted person's parents, the highlighted person's spouse, the spouses's parents, and the highlighted person's children. Getting visibility to the whole family on the screen is the key to avoiding the kinds of duplication that you are talking about.
You would add children or parents to an existing person pretty much as described above. First, go into Family View, highlight the person, and do whatever it takes as described above to move them into the father or mother position. If the existing person can't be moved, then right click them and Add Parents or Add Spouse or Add Child as appropriate. As soon as you can, move them into the father or mother position so you can see the whole family at the same time.