Don't warn about duplicate identical attach links

[GiovanH]

• I have searched the issues (including closed ones) and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.
• I am willing to lend a hand to help implement this feature.

Feature Request

By default, any time an {attach} link tries to overwrite an existing file, you'll get a warning message and Pelican will treat the link like {filename}, just linking to the existing file.

This causes extra warnings if you have multiple attachments in the same document. For instance,

![]({attach}avatar.jpg)

![]({attach}avatar.jpg)

seems like a valid input. And Pelican's behavior is exactly as expected; the file is attached on the first link, and subsequent links point to that same file.

The problem is the warning messages. I'm not sure if Pelican has a system to keep track of this sort of thing internally yet, but if an attach link has to be coerced to a filename link, that should happen silently if and only if the source files are the same.