The uni-brow appear only in the GECK and the eyebrows look perfectly normal in-game. Have you tried starting your game to see in the uni-brow was seen?
The problem I kept having was both in game and in the Geck. What I found was when ever I added a folder labeled "hair" in my mesh or texture the game would look for the eyebrows. Since neither the texture for eyebrow and/or the mesh for eyebrow was in the folder it would create the problem you see in the image. So I kept putting in custom hair and then stripping it out until it dawned on me to open the Fallout BSAs and pull out the eyebrows mesh&text and install them. Problem solved.
I've seen other people ask for a solution to the same problem in a couple different forums so I figured I'd share what I learned.
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it could be easier to know what to do
I have no idea what you mean. Either download with the manager or download by hand and install it.
I always download by hand but I've been doing since this since 2010. (Oblivion, F03, FNV, Skyrim, and F04)
I've seen other people ask for a solution to the same problem in a couple different forums so I figured I'd share what I learned.