If you edit a post, and access the media library either via Add Media or the Featured Image link, then delete an existing image on the post and try re-uploading it, you may find the same old image shows up. This is due to your new file having the same file name, and the old image not being deleted from the main Media Library (deleting it from the post editor only removes it from that post).
Option 1: Make sure the new file has a unique filename
Rename your new image file, then try the upload again.
Option 2: Ensure the old file is completely deleted
- Click on Media in your admin menu
- Locate the image there and delete it. Also ensure no duplicates exist and delete those too
- Return to your post and upload the new image