Show warning during uploading paper which already exists

[dipamsen]

The Problem

Many times, people (unintentionally) upload papers whose PDFs are already available on IQPS.

Possible solutions

This can be prevented by showing a warning message on the Upload interface, when a similar paper already exists in the database. Additionally, there may be a link to the similar paper, so that the uploader can verify that the paper they wish to upload already exists.

Note that this should not block such uploads as there are legitimate reasons to do so, e.g. uploading a better quality PDF for a paper, or in the case where the original paper is mislabelled, etc.