I want to prevent duplicate attachments from being uploaded to the space such that the existing attachment is updated and there is a "source of truth". I also don't see a way to link an attachment from a different page easily (I thought this feature was here in the non-cloud version).
I think there is no way to check duplicate attachments in Confluence, you need to use a plugin for that.
@Byron D Aguilar Welcome to the Atlassian Community.
With regard to linking to an attachment from another page:
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