Dallas here from the Social Care Team at Shopify.
That's great that you're asking those questions to hopefully prevent you from dealing with issues down the road. With that said, however, by the sounds of it I think you're going to be alright. If you got these dresses from dropshipping suppliers like AliExpress then all the dresses are able to be sold on your store no matter who's also selling them. The reason that you're probably seeing other stores with those dresses would be because they are also sourcing the dresses from that dropshipping supplier.
I say this, but that would still ultimately depend on where you're getting the items. Are these dresses from a supplier? If you're designing them as a copy from another store then that wouldn't be ok, but once an item goes onto a catalog for dropshipping then it is available for anyone to use. Does that help to clarify a bit?
If you want to send me some screenshots of where you're seeing this dress and where you sourced the dress from I'd love to take a closer look for you. All that I am saying right now is speculation from guessing the situation, but I can't say for sure unless I have some more concrete information. Links are also great as well so if you have any links to share please let me know.
I hope this was able to help, but I look forward to helping you further.
Unfortunately, that is an issue that I'm not sure I really can help with. Since I'm not trained in any legal, I suggest that you reach out to a local legal professional to see what they have to say about this particular instance.
I'm sorry that I can't be of further assistance.