There are plenty of ways you can improve the quality of the site, especially if there are other users that feel the same way as you.
If you notice a duplicate post, politely inform the user that the topic has been discussed before (or is still ongoing) and provide a link to that discussion. Encourage them to post their comment in that thread and contribute in that location. Explain that it will help to keep everything in one location, instead of scattered around the site.
Hi! You make some valid arguments in your post. Would you mind reposting those in our ongoing discussion on that topic [here]. That will keep the entire discussion contained in one location making all of our lives easier because it won't be scattered every where around the site.
If you are noticing low quality posts, perhaps the user just needs a simple nudge to provide more context or better arguments or even spending time to right click on those red squiggly lines and correct some spelling mistakes. A simple message can really go a long way.
Welcome to [site]. It looks like you have a good topic to discuss here, but there are some aspects that are making it hard for us to understand you. Could you correct some spelling/provide some more context/cite a few of your sources so that we can better understand what you are telling us?
If there are other users that feel the same way as you and want to help improve the quality, it is worth talking to them and developing a way to handle this type of stuff. There are a few simple options - such as the polite notices from above and then disengaging the user until fixes are made. This encourages the new user to post in the appropriate threads and lets their duplicates slowly trickle away and keeps the originals active. It can also shows that the community won't engage in a conversation with a user that can't present a coherent post. By having multiple members follow this action, you show that it is a community wide effort to improve quality.
You mention that this is a start up. My assumption is that the product is still being actively developed and the goal of the developer is to build a community of some kind behind this product. It is probably worth while to approach the developer(s) and inform them of the lack of moderation and the perception you (and other users) are starting to develop. Building the community is as important as the product itself, if not more so. The developers/owners need to be active in the community and understand the audience that is using their product.
When you approach the developers, it is a good idea to mention the actions the community has already taken. This shows that the community is serious having high quality. It should also show the developers that they need to pay more attention if this type of organization occurred and they hadn't noticed.