Below sentences is confusingly structured.
It is not uncommon for the person being sexually assaulted to have no physical injuries or signs of their assault.
Is the meaning that it is common for there to be physical injuries/signs of assault or that it is not common for there to be physical injuries/signs of assault?
Think of it as a continuum:
Then the interpretation of 'not uncommon' means more often than 'not uncommon' but perhaps not 'common'.
But even that sort of view requires reader and writer having an agreed upon scale and what the gradations mean. Without that, the writer and the reader may quantify the territory covered by 'not uncommon' differently.
Another way to view it is that uncommon by itself means 'not common' so 'not uncommon' could be written by extension as 'not not common'. With the double negatives cancel leaving you with 'common'.
It's a poor choice of words because 'not uncommon' doesn't tell you what the prevalence of the condition described is, it merely tells you what it is not. At a stretch, it could mean 'very uncommon' (which also isn't precluded by 'not uncommon' literally. It could also mean 'in all cases' (because that also isn't precluded by 'not uncommon'). Its interpretation is very much dependent on the reader and that isn't desirable in clear communication.
It would have been better to say 'common' or even better to have cited some sources that could put some actual quantization of the frequency of the state discussed.
In standard/usual usage, the phrase "not uncommon" typically means something to the effect of "more frequent than uncommon". That is, it occurs too frequently to be considered "uncommon", but not necessarily so frequently as to be labelled "common".
So in this context, it means that it occurs frequent enough to not be considered "uncommon", or simply put "common", for the person being sexually assaulted to have no physical injuries or signs of their assault.
You can read more about "not uncommon" vs "common" here
