The vast majority should ideally be unique; the minimum acceptable level is 85%.
Let the contributor know they've been accepted
Get their full name
You'll need this to create the author profile on your website.
Get high resolution images if needed
Depending on the column width of your blog, you'll have a minimum resolution for images. Make sure they've attached them, and send it back for edits if they haven't.
Get their bio
You need their bio to fill in their author profile fully
Paste the article into WordPress
Paste HTML data into the 'text' view, and Word/Google Docs content into the 'visual' view.
Check for filler words
Read and refer to this article and remove all filler words from your work; if the word is not serving a purpose and could be cut, it probably needs to go.
WordPress' 'proofread' function can catch these, but don't rely on it.
Make sure it's easy to read
Anyone (including your gran) should be able to read this content. It shouldn't matter what background they're coming from; they should be able to keep up with the language you use and the arguments you make.
Double check all links
You can use the Check My Links extension to do this automatically in a few seconds.
Make sure that Amazon links are affiliate links.
If you want to add more links, check the Airtable tab and try to go for Red Hot or High priority keywords.
Clear up any formatting issues
If you're pasting from Word or Google Docs, you're going to have a bad time reformatting in WordPress. Make sure that the line breaks are even, and that there are no strange HTML tags in text view.
Break it up with their images or your own
It's good practice to include images. If they've not provided enough, either send it back and request images or just make/attach ones you've found yourself.
Ensure the post is scannable
This can be achieved by using sub-headings, breaking up paragraphs with quotes, bullet points, etc.