I can see occasionally duplicated files created on a single review. I was able to replicate this issue by adding several changes to the same review at a time.
Scenario to reproduce:

1. create empty review
2. add content, browse for changesets
3. pick up a branch/commiter so you can see several changesets with checkboxes. Changesets should be touching same files.
4. quickly check several checkboxes, don't wait for previous to complete when selecting next
5. notice some paths were duplicated on the review

The problem does not exhibit itself when checkboxes are selected one after other, when waiting for the previous one to complete.

Most likely same issue can be caused by several REST actions or API calls to add content to review.

Edit: This might also trigger for other cases, unrelated to concurrent requests (see FECRU-3438 and https://atlaseye.atlassian.com/cru/CR-BAM-5260)