Several of mine have them and most almost disappear when they bag up. One sheep's stays about 1/5 the size of her real nipples and have not caused any problems. They could get in the way if you are hand milking but are not a problem with machine milking (for me anyway)
Odd fact of the day, humans grow extra breast tissue, even sometimes extra nipples, all along the "milk line".
These often develop when a mother becomes pregnant. If it's the same with sheep, your ewe might not have had that extra nipple until she got pregnant.
(How do I know about this? When I got pregnant with my son, a "lump" formed in my right armpit. February 14th, I went and got an ultrasound, to make sure it wasn't cancerous. It was *drumroll please* extra breast tissue. I told my husband I grew and armpit boob for him for Valentine's Day, LOL! I thankfully never developed a nipple in my armpit, and the lump went way down once I started nursing. But, there are women who develop breast tissue on one pregnancy and then a nipple on the next, and the thing even produces milk. I was told that those women who gain an extra nipple just don't milk it and it eventually ceases to produce.)
But, here’s where she got it wrong: My milk finally came in for my first born, and while I was nursing him, I asked my husband if he’d turn up the air conditioning because I felt like I was sweating profusely. It was then that I looked down to see that no, I wasn’t sweating, but what I thought had been the tiniest of brown freckles was instead a nipple that was currently dripping milk down into a puddle on my shirt. Yep, sign me up for the circus people! Not only do I have a third boob, but it lactates. Lac. Tates.
I had to nurse with a towel in my armpit, and while most nursing moms got to walk around with just absorbent breast pads in their bras, I got to do that and wear a huge band-aid on my armpit. Just so you’re all aware here, my son was born in June. Tank top weather. It was fabulous.