Extra line breaks

#1
I've noticed that if I compose a post here, and copy from the text box (not the post itself), when I cross post to another site, I get extra line breaks and have to fiddle around deleting them. (If I cross-post to here from another site, I don't get that.)

Any ideas? (The reason for doing this is because I'll post stuff here as a single post that would be split into multiple threads/posts in other sites.)

Mac OS X High Sierra, using Chrome.
 
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