Applying a paragraph style causes text to appear duplicated on screen, but it's not really.
CC 2018, OS High Sierra, both most current, brand new install on a brand new iMac Pro.
If I apply a style using a key command, 90% of the time the text from the cursor down goes crazy, looks duplicated. But if I hit the key command again, or click off the paragraph, or scroll, it goes away and is normal. So it's not really duplicating anything as far as I can tell. I screen captured exactly what it looks like.