They eye stripe and white neck are pretty indicative of a males, but that actually can vary by breed. I have two girl ducks that have white lines on their necks, and they are girls. Do you happen to know what type of ducks you have?
The best way, honestly, for identifying the sex of a duck, is by sound. The girls are generally louder than the boys--even as ducklings. And, as they get older, the girls will make a distinctive "Uuht!" sound, and the boys will make a softer, almost wispery "wah" sound.
Majestic Water Fowl Sanctuary has some great information on sexing a duck this way:
http://www.majesticwaterfowl.org/artquacks.htm. Click on their audio links to hear the differences. Then go and pick up your ducks, one by one. If it's a girl, it'll likely say a loud "Uht uht!" (unless they've been hand-tamed by you).
As for whether another male would be a problem, the answer is sadly yes. When you have more than two drakes, you want a ratio of 2 drakes to at least 10 girls. If the drakes are super polite, you can probably get away with 2 drakes to 8 girls. But, any more drakes than that, and the boys will pick on the girls too much. When I had too many drakes, they wore off the neck feathers of my girls, and--with their favorite lady--even wore off all the feathers on her back!
In the end, I had to put all the drakes in their own yard and house them in a kennel away from the girls at night. We didn't have the heart to eat the drakes my kids had raised, so we just kept them separated. The boys kept each other company, and the girls got a break from the boys. If you don't want to eat the boys, you could give/sell them (and they'll probably become someone else's dinner), or put them in a duck tractor to eat slugs and bugs around your garden. And, in the fall and winter, you could probably move them back with the ladies until spring--and mating season--comes around again.
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