Hey guys.
I had the same problem with Digg and I think I have found a possible solution for it.
You may have your Custom Permalinks Box set to something like this:
https://example.com/%category%/%postname%/
What this does is submit to a social site something that looks like:
https://example.com/https://example.com/category/somepost
The problem is that it’s picking up the entire string, so it’s adding it on top of the URL that’s being submitted already.
To fix this make sure that you set the custom permalink only to:
/%category%/%postname%/
Now it should work with Digg.
Hope this helps!