Hello,
Are you saying that: When you use Shortcode for inserting a Block its printed twice, one in the post content and one in the header/footer?
If yes, then I believe you're doing it wrong!
Why you assigning a Block to a page while you're inserting as Shortcode?!
If you need to insert a code Block into a post content, just do the following;
1. Create Code Block
2. Add your code there (JS Code above)
3. Save Block
4. Edit the Post you like to insert the Shortcode
5. Select the code block from the TinyMCE CJT Shortcode buttons list
6. Save post.
NOTE: We didn't assign the Code Block to the page as its already inserted as Shortcode.
Short code blocks doesn't use assignment.
Hope that helps!
Regards,
AHMeD