WDYD - smashed eggs? UPDATED: Pic!

I have three birds of potential laying age - an EE, a Pekin duck, and a Swedish duck. Saturday morning I found a large, whitish egg that I think came from one of the ducks. Nothing yesterday. This morning when I went out there were two eggs - one about the size of the first and one smaller. Both white. Both smashed on the ends. I'm not sure if the shells are too thin or if someone is pecking at them or if they simply got stepped on. They haven't gotten the idea of laying in the nest box yet, so the eggs are just in the enclosed part of the coop.

The feed store was out of oyster shell when I went but I did pick up layer feed and have fed them. I was planning on stopping somewhere to buy golf balls on my way home from work (I heard throwing those in the coop can help discourage pecking?). Is there anything else I can do? I believe soft shells can be a sign of low calcium - since I can't get oyster shell should I stop by the pet store and get some of the supplements to dust the feed like they use for lizards?

TIA!

Update:

So the egg laid the next day was totally normal. Then the day after (today) I found this:


It's mostly membrane, no shell? There was a second one in the coop, very similar, but smashed. I'm wondering if it is the same bird laying both eggs and popping both out at once is causing a problem? They are on layer feed and I've been giving high calcium treats like cottage cheese.