Looks like it is still an issue based on what I have detailed below.
When I activated the Publicize feature to add my Facebook and Tumblr accounts, every post I made an update to (56) a few days later was re-published on my accounts even though I manually shared them earlier.
Today, I added my Twitter account and made a change to one of my post and it immediately posted the updated post even though I had previously shared it back in January. However, minutes later when I made another change to the same post, it did not duplicate/ re-publish the post to Twitter. Nor did it re-publicize to Facebook or Tumblr. I also checked my RSS feed today and did not see that the post was re-publicized/posted twice.
This is what I think is happening:
When you add an account to Publicize, if you make any updates to previous posts that were manually shared, they automatically will be publicized and duplicated to the account(s) you added. Any additional updates to the same post will not be re-publicized but it will for others.
Which is unfortunate because looks like what we need to do is deactivate Publicize when making initial updates to any post after adding new accounts so it does not duplicate the posts since I find no matter when I go in to update, it is being republished/duplicated.
I agree with swaroop and think it would help WP users if there was a feature we can choose to request that previously published posts not be re-publicized when adding new accounts to eliminate our having to physically turn off the feature every time we make an update.
As for posts imported from another blog, I am not familiar with that so I cannot make any suggestions. But if I ever have to try this feature, I will let you know what issues I may encounter.
The account in question is joaynn510.