With the RSA encryption algorithm, is there any guarantee that two different entities will not be given the same private keys?
When generating a 1024-bit RSA keypair, one of the first jobs is to pick a pair of primes p and q such that the product of those numbers (i.e. p multiplied by q) is 1024 bits in size. The simplest way to do this is to pick both values to be somewhere around 512 bits in size each, as 2512 × 2512 = 21024. Note that I'm picking 1024-bit as it's now considered to be the lowest acceptable key size, in which you're going to have the highest likelihood of prime collision.
Now, if we assume that only 512-bit values are used for p and q, we can now estimate how many primes there are in that range. I discussed this in an answer to another question about RSA, but I'll re-iterate here. The prime counting function allows us to estimate the number of primes in a given range. It isn't really defined, as it's just an estimation, and there are various ways to compute it. The simplest and roughest way is simply π(x) = x / ln(x), where π is the PCF and
ln() is natural log. As I explained in the linked answer, there are likely somewhere around 1.885×10151 prime numbers in the range 2512 < n < 2513.
This means that, assuming an ideal implementation, the chance of picking equal p or q values in two independently generated RSA key pairs is roughly one in...
That's slightly less likely than winning the jackpot on the UK national lottery 21 draws in a row (13,983,81521 = 1.143×10150).
This is all assuming that the implementation is ideal. However, this isn't always the case. For example, there used to be a bug in OpenSSL that caused uninitialised stack data to be used as the random pool for generating RSA primes, which meant that the actual number of possible outputs from the RNG was quite small. The resulting key pairs are now blacklisted from most implementations to prevent exploitation. Another example is that in many embedded devices (e.g. firewall appliances, routers, etc.) certain cryptographic values, such as SSH session keys, are generated early on in the system boot process, when there isn't much entropy available. This leads to a situation where the number of possible generated keys is significantly lower than you might expect, potentially leading to prediction attacks.
The tl;dr is that the key space is so large that randomly selecting two equal primes (p or q) when independently computing two RSA key pairs is phenomenally unlikely.