I would have guessed the SHA1 hash check happens server-side. I've definitely ran into the warning myself using the native Special:Upload interface. That tells me the hash values probably don't match, and whatever image Flickr2Commons is trying to upload must have subtle differences with the duplicates already on Commons. I'm not sure how this could be resolved without machine learning. MusikAnimal (WMF) (talk) 16:48, 30 January 2023 (UTC)[reply]
Yes, there is a server side check. It produces a "warning" (not an "error"). In case of an error the file will not be published. In case of a warning, the upload tool can decide to stop the upload or to go on and publish. It is possible to compute the SHA1 and check against the server, than not upload, if it is already there. Or it is possible to upload a file, than check for the warning, than stop the upload, if there is a duplicate. You could even do both: Check before uploading and check again before publishing the upload. F2c only checks for an identical filename (not an identical flickr id, but for an identical filename including the flickr Id. If the file gets renamed (or was uploaded with an other filename scheme (like Upload wizard does) the check by f2c will fail. C.Suthorn (talk) 22:57, 30 January 2023 (UTC)[reply]
What are the benefits of Flickr2Commons compared to the Flickr-import functionality in UploadWizard? It seems like it might be better to incorporate anything missing there, rather than having multiple things do the same work. SWilson (WMF) (talk) 01:01, 8 February 2023 (UTC)[reply]
When i still tried to use the UW it allowed to upload 150/500 files in one go, but i never found any way to upload more than 100 files in one go without the webbrowser crashing. Also in the second go of uploads it always said, the first file of the second go was already uploading and it falsely claimed, it was more than 150/500 files, if the sum of the first and second go was larger than 150/500 files. C.Suthorn (talk) 17:55, 16 February 2023 (UTC)[reply]
Support I support anything that reduces duplicate files. It seems to me that the Upload Wizzard and other upload tools should have the same/similar checks. JopkeB (talk) 09:43, 11 February 2023 (UTC)[reply]
Support As a frequent user of the tool, despite being very careful with it, I myself have erroneously uploaded over a thousand duplicate files. Vis M (talk) 06:39, 16 February 2023 (UTC)[reply]