Using numbers as a way to interpert the cards is a geat way to understand them. You asked several really good qustions.
1. What does a number mean when it appears more than once in a reading? I means that the energy of that number is prevelant in your life in several different areas. Let's say that you drew the Three of Cups and the Three of Pentacles, along with the Empress (who also carries the energy of the number three). You would be dealing with other people, and different avenues of resources. The Cups talk about relationships (personal and busness), as well as intuition. The Pentacles indicate that your daya to day material security (both home and at work) is dependent upon your working with other. The Empress is there in a spiritual way to add nurturing energy.
2. Does a numbered card carry less "weight" than a Major Arcana card? The Major Arcana is the spiritual part of our life. It is something that we have no control over. The Minor Arcana tell us how the Major Arana are working in our life - what areas they are affecting, and how that affect is taking place. With the Minor Arcana, we can take a look at what is goin on and make changes, where we cannot with the Major Arcana. The Court cards talk about the people and issues in your life, although they can also represent a specific part of yourself.
3. Most people use the numbering system where Page = 11, Knight = 12, Queen = 13 and King = 14. If you are working with a specific esoteric system that is different, then you need to follow that system.
4. Is there a system for reading numbers that are next to each other? Yes, there is. Generally speaking, like numbers strengthen, and unlike numbers weaken. Two odd or even numbers together strengthe each other, where an odd and an even number weaken each other. If a number has an odd number on one side, and an even number on the other, then it is considered to be neutral.
A good site for a general review of the meanings of the numbers is http://www.bewitchingways.com/tarot/numerology.htm.