When using Citoid to expand a DOI or other identifier into a full citation template, the link to the DOI or identifier target often gets added to the |url= parameter. The citation then has the same information twice and the URL is redundant. Guidelines and documentation discourage this, for instance https://en.wikipedia.org/wiki/Help:Citation_Style_1#Identifiers :
When an URL is equivalent to the link produced by the corresponding identifier (such as a DOI), don't add it to any URL parameter but use the appropriate identifier parameter, which is more stable and may allow to specify the access status. The |url= parameter or title link can then be used for its prime purpose of providing a convenience link to an open access copy which would not otherwise be obviously accessible.
Other tools and bots like citation bot on the English Wikipedia will later remove such URLs, but it's better to not add them in the first place. When the user actually inputs an URL, such as the DOI preceded by https://doi.org/, it's debatable whether that should be retained as URL: you may want to keep it if you consider the users are so sophisticated as to provide a bare ID to use the corresponding parameter and the full URL to (also) use the URL parameter. I personally believe most people use them indifferently just to provide the ID in whatever is the most convenient way at the moment, so it doesn't matter. It also doesn't matter on the English Wikipedia anyway because eventually such redundant URLs will be removed.
This task is arguably a first step before T174540, although T174540 could also be implemented by just putting whatever is the target of OAdoi.org expansion (which make the URL parameter redundant in 50-70 % of cases rather than the current 100 %).