It's just easier to keep track if you collect them every day. I put dates on the egg cartons (we have several layers) so I know which are the oldest.
No sanitizing is necessary. As long as the duck is healthy and their environment relatively clean (that includes some poop in the bedding, just not ancient poop), and you are not allergic, it should be fine.
Here's a test for freshness - place the egg in a clear container that holds about a cup and a half or more of water. Pour cold water over it.
If the egg floats, it's a goner, do not eat it.
If the egg lies on its side on the bottom of the container it is fine, may be about 2 weeks old or so.
If the egg stands up tall on the bottom of the container it is rather fresh and may not be easy to peel if you hard boil it.
I crack eggs into a clear custard cup so I can fish out egg shells if I get a little bit of shell in there.
Duck eggs have a protective coating on them called a "bloom." I have been advised not to wash the eggs off - it damages the bloom. If there is a little schmutz on the egg, just wipe it off with a dry cloth for that purpose. Then you can rinse it off well just before opening it when you cook it.