Thank you all so much for your replies, you've been so much help!

Well I went back to the person who sold us the ducklings yesterday but he wasn't there. His son was there instead who was present when we purchased the ducklings the other day. I explained to him what happened and he said that his father had probably made a mistake as they didn't have any goslings 3 weeks ago when we bought ours and that he might have sold us Muscovy ducks. But he still wasn't sure about them. Well he should have corrected his father when he heard him selling us the ducklings for goslings. :/ He charged us 10 euros more per duckling (which is alomost 25 $). Anyway he wasn't willing to give me my money back but he offered to take them back next week when there will be goslings available and give us 2 goslings instead, but we don't intend to give them back. We've loved them and they've loved us I think. They start peeping when they see us approaching their pen and they want to be patted.
Now talking about their sex, I guess we'll have to wait a couple of weeks more to hear what their voices will be. The yellow one who was almost half the size of the brown one when we brought them home, has grown as big as the brown now and is much more "noisy" than the brown one. Will there be a problem if both of them are drakes? I mean is there a chance that they will start fighting?
Thanks so much again for the help!