In Dynamic Post Boosting campaigns, unfortunately it is not possible to exclude posts automatically if they're created by choosing existing posts. You need to do that manually. 

You can exclude posts by going to the "Post Filtering" section and using "Excluded Post IDs". The post ID can be found from the post's URL. 

For instance, if the post’s URL is:
The Post ID is:

If you created the post by using the create new ad option, as it creates dark posts for your ads, you don't have to exclude them from other campaigns.

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