Include a link to the already existing item
For error message saying the document was rejected because it already exists in the account - if in the error message it can include a link to the previously uploaded document.

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Shannon McMonigal commented
This feature would be SO helpful in cutting down the time wasted searching for where the document went - generally I'm re-uploading it because the supplier is chasing payment or has sent a statement and I can't find any trace of it in the system - so knowing where the file went is crucial to know if it was coded incorrectly.
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Nick Yatracos commented
When uploading a duplicate document it's very handy that Dext rejects it but sometimes it would be very helpful if Dext provides a link to the existing document as you may need to double check something or updated it