You only need to handle collisions between any two objects once, so you have a symmetry problem here. If you first handle "A collides with B", then you must not handle "B collides with A" later on. And, by definition, an object can never collide with itself, so you must exclude this case.
I don't know how you would go about doing this with foreach syntax, but the basic idea is that if you have N objects, your loop looks like this:
for A from 0 to N - 1 do
for B from A + 1 to N - 1 do
if (Object A collides with Object B) then // handle stuff
This will ensure you never handle the same collision twice, and also excludes same-object collisions.
The slowsort algorithm is a perfect illustration of the multiply and surrender paradigm, which is perhaps the single most important paradigm in the development of reluctant algorithms. The basic multiply and surrender strategy consists in replacing the problem at hand by two or more subproblems, each slightly simpler than the original, and continue multiplying subproblems and subsubproblems recursively in this fashion as long as possible. At some point the subproblems will all become so simple that their solution can no longer be postponed, and we will have to surrender. Experience shows that, in most cases, by the time this point is reached the total work will be substantially higher than what could have been wasted by a more direct approach.
- Pessimal Algorithms and Simplexity Analysis