[Generic Man One]

Also, since I'm here. I was curious and worried about polled ewe's headbutting each other. While I understand it's not common, should I be worried about potential health issues?

[SallyintNorth]

Butting, whether polled or horned, is usually about dominance.  Settling the hierarchy when sheep are regrouped, arguing over food, chasing another ewe away from my lambs, that sort of thing.  Generally it's quite short-lived, as the boss one gets established quickly.  If it continues it may suggest some underlying stress factor - too little space, not enough food, dogs too nearby, etc. 

[silkwoodzwartbles]

Reasons for a ewe to reject her own lamb could include mismothering at birth (eg if another ewe licks it first she may not think it's hers), a difficult birth causing trauma, something wrong with the lamb that means she doesn't think it worth bothering with, or a lack of mothering instinct (also known as a strong desire to upset the shepherd and wind up in the freezer). What is the situation behind your question?

SallyintNorth has answered your headbutting question really well

[Bywaters]

why do they reject one / both ?

Flippant answer is because they can !

A multitude of possible reasons; first timers with twins possibly concentrate on one and forget the other? perhaps an inbuilt reaction if they have not much milk ?

Inside lambing or outside ? Might make a difference, we lamb outside unless we know of potential problems.

[Mamohau]

We have a Soay x Shetland, experienced mum, and a couple of years ago she rejected her lamb at about 2 days, after we'd dipped the umbilical cord.  We tied her up, surrounded by bales, and managed to get the lamb drinking from her (hours later), and then once she'd stopped kicking him we released her, still in the bale pen and monitored his weight for 2 days. Then we released them into the barn and then back with the others.  Our first lambing and we got a rejection; fast learning curve, calling shepherds for advice while they were all at work!
We weren't sure if it was the iodine smell, our handling (although she knows us), or just some potty thing got in her mind, post-partum depression! She was fine last year.