A CSPRNG is expected to return every possible value of the image space with a probability indistinguishable from a random distribution.
Leaving certain results out would make a bad design. Think of dice. If you roll 1-1-1, you would not think that this is not random enough and sort it out - You just generated that entropy with certainty that it is real.
Doing so with an PRNG would compromise it's security, so no, there is nothing filtered out, hopefully.
Side note here: even if the probability is very small (so small as to be negligible): you are not supposed to reuse the same IV on the same key with AES-CBC, as to not leak anything if your files share structural information
Other than that, you should be fine with your approach. Every call to
SecRandomCopyBytes you may consider a dice roll, for all you care about, as it's the best entropy you are going to get.
If your aim is not only confidentiality but also data integrity, you would be better off using another mode of operation as pointed out in the comments. I know, iOS is not very generous in giving much choice there, though.