There is a very non-0 chance that two nodes can generate the same address, and nobody would be none the wiser.
There is not a VERY non-0 chance.
you'd almost 99.999% be guaranteed to never find a single collision.
Nor an ALMOST.
There has NOT been one SINGLE documented crack of a conventionally generated 160 bit SHA-1 hash. It could happen (if god played dice). Perhaps someone will come up with a fantastically clever new non-brute force algorithm. Perhaps computers will compute faster than currently understood physical limits (such as infinite quantum states).
1 - (number of blocks, 135483) * (new addresses per block, 1 to 10) / (key space, 2^160) = 1 - 10^(5 or 6) / 10^49
99.9999999999 9999999999 9999999999 9999999999 9 % chance that it's not gonna happen
(give or take a 9, minus the chance of god playing dice, new published algorithm crack, or yet unknown technological innovation)
The only real protection (if you want to call it such) is to have many addresses in your wallet with all your Bitcoins spread among them. If/when an address is compromised you will potentially lose a little rather than everything.
No. You'll then have only divided a seemingly infinitely improbable chance by a tiny finite number (number of your wallet keys with value). You can remove a '9' from my estimate above. If someone can crack one hash, then they can probably crack a huge number of them. Though not putting "all your eggs in one basket" is good advice for other reasons.
EDIT: strike-outs are mine.