I have a problem in two steps.

Step 1:
I have one ID column and one column with numbers. The same ID may appear
several times, but all ID's must not be included (depending on other
variables) What I want is to use something similar to the LOOKUP function,
but that returns All values of the given ID (not just the first one) and add
them together. What function should I use and how?

Step 2:
I want to create a third list that includes only the ID's which were
included in the ID column, the sum of the numbers that they were connected
with and preferrably sort them in falling order based on the numbers column.
How do I do this?