Vascular occlusion is known to be a blockage of a blood vessel with a clot or embolus, while stenosis is an abnormal narrowing of a blood vessel due to sclerotic changes. Taking into account this fact, we may suggest that Occlusion and Stenosis in the context of ICD10CM are NOT simultaneous conditions (I65 semantically implies OR between them).
So, if the vocabulary team map I65 to both 374371 1055001 Stenosis of precerebral artery AND 443239 266253001 Precerebral arterial occlusions, this will return obligatory both of them during a study whereas one of them never happened.
Also, it is not allowed to map them only to one concept because this leads to irrevocable data loss.
According to the mapping rules, such ICD concepts are mapped to the closest common ancestors (as 43022059 473449006 Disease of non-coronary systemic artery and 255919 118254002 Finding of head and neck region for I65), which can help to embed them into SNOMED CT hierarchy as SNOMED Extension concepts in the future (read more).
But now the best idea is to exclude I65-like concepts from the study.