As a bit of confirming data that it is an SP problem:

–I deactivated some other plugins that had been updated and the double-comment problem persisted.

–When I delete a comment that is appearing twice in WordPress, a copy of the comment persists in SP.

The simple problem with your assertion is that the blog linking plugin has not received an update since July 20th 2014 and ALL of the code pertaining to blog linking behaviour is contained within that plugin.

So – what else has been subject to an update at around the same time?

Only three plugins were updated before the problem appeared. As I said above, I disabled the other two, one at a time, and checked to see if double comments were being posted. They were. So the only thing that has changed other than those two since the problem appeared is Simple:Press.

However, just disabling SP to test it is not so simple. Is there a way to disable Simple:Press without also disabling comments? When I disable SP, it also makes the comments disappear.

Definitely strange but I can’t see any relation between SP and WP comments. I’m noticing a couple of jquery errors on your page though – just out of interest have you tried one of the default WP themes?

do you have double comments in your db?  you can look in the wp comments table and sfposts table…

your test would only tell you something if the comments were appearing on display only.. if they were actually doubled on saving, you would still see two…

perhaps you can try disabling the other plugins that were updated and then make a new comment and see if doubled up…

no wp update at same time right?  and you did not update our sp blog linking plugin, right?

As I said above, I have disabled other recently updated plugins. Double comments still appear.

I did not update WP at the same time. I did not update the SP Blog Linking Plugin at the same time, either.

The comments are not duplicated in the database.

I have confirmed that it is absolutely SP 5.5.5 that’s causing the problem.

I duped to a staging server that is identical in every way to my production server, except, by necessity, for the hostname. (This is done through my hosting provider, WP Engine.)

I disabled by WP extensions one at a time, in reverse order of recently updated, and, after disabling each one, forced-refresh the page, and posted a new test comment. Doubling of comments appeared each time. In the end, the only plugin left was SP and double comments were still appearing.

Then I changed the theme to the generic WordPress 2015 theme. Double comments still appeared.

Then I started disabling SP’s own plugins one at a time until the only one left is Blog Post Linking. Double comments still appeared.

Then I disabled the Blog Post Linking plugin. All the double comments disappeared from the post! When I reenable Blog Post Linking, all the double comments reappear under the post.

Attached is a screen grab of my Blog Post Linking settings. Does anything look like changing it might have an effect?

what about if you changed the option to display in comment block vice in standard comment block?

Looking through the wp code, it appears that in 4.1 they changed how the comment query works… and this might have a knock off…

however, that version of WP was released in December… so if indeed the case, would have been an unreported issue since the WP 4.1 release…

So have opened a ticket to see what broke in blog linking in the wp 4.1 release…

This is strange, but after a whole bunch of noodling, it turns out the double comments only happen to posts where I change the spelling of the title. Just the title, not the permalink.

This post — where the problem first appeared — is where I have been testing double comments:

http://www.waywordradio.org/le-petite-voyage/

However, I was not testing any other posts — and after doing so (realizing that would be proper), it turns out that none of the other posts that I have tested are showing this problem of comments doubling. I see nothing at all unusual about the post! The HTML is fine. There’s no hidden code. No embeds or iframes or anything like that. I couldn’t figure it out.

But one thing that was different: I changed a typo in the title and then saved that change. That was it.

I’ve now tested it in a bunch more posts. In all cases, except for one, simply changing the title — either making it longer, shorter, or the same length but changed — and then updating the post, and then leaving a comment means that any subsequent comments are duplicated.

The one case where it did not happen cannot be duplicated. I added a single letter, updated the post, and then left a comment, and the comment was not duplicated. Then I went in and edited again, only longer, and this time all comments were duplicated.

interesting.. I am able to replicate the duplicate comments just by simple adding of comments to the wp blog post when using your settings…  the duplicate goes away with the change I mentioned…

but still something we need to look at and see what wp changed in December with the 4.1 release…