The histogram implies this behavior started in 55.0.2883.18

I believe the flow is this:
- User clicks download, ultimately resulting in a call to OfflinePageDownloadBridge::SavePageIfNotNavigatedAway
- That function always schedules a request with the RC, and grabs the request ID.
- That function also asks recent tab helper to save a page with the same request ID.

If for some reason they both complete, then ALREADY_EXISTS would be the result.

Interestingly, due to the archiver design, we will overwrite the file with the contents of the second download invisibly.

looks like https://crrev.com/02d38c0b50c55dadb98039dbc58a70ad2c7c8a15 is the offending commit.

Might be a duplicate, but has additional info.

This might be saving a page over a page of the same name failing.  Now that we save each page to a GUID name, we should see a marked decrease.  

This is likely a longer investigation, and perhaps not a good candidate for the fixit.

 http://crbug.com/704726  will fix this.