I have a category page that if it has one product on it, it will redirect to that particular product page using a 302 redirect. However because of this, if I look in google webmaster tools, it is complaining that I have duplicate titles on the category page and the product page it has redirected to.
What is the correct way of redirecting this page without the duplicate page titles being flagged? I can't do a 301 as there may be 2 products in future and so the redirect may be removed.
From SEO perspective, redirecting the category page to the product page is like shooting your own foot.
By having separate pages, you have more chances to rank for more keywords.
It is understandable to use a 302 redirect to get users directly to the product but as you can see it causes all kinds of consequences.
If you are worried about user experience, you could using a Javascript to redirect users and block search engines from accessing the Javascript - so search engines don't get redirected.
However, you need to be careful with JavaScript these days because Google has started to execute them pretty well - thus make sure to block the files.
here is a blog post that talks about Google can now execute Javascripts. http://www.rimmkaufman.com/blog/google-crawling-javascript/13052015/
In fact, you can also use javascript redirect to do a 301 redirect https://digitalreadymarketing.com/301-redirect-javascript/
NOTE: Technically speaking, it is not a 301 redirect since Javascript is executed at browser page level and the status code will remain the same. However, Google will pick it up as a 301 redirect.
Here is more tips on how to deal with seasonal / out of stock product pages https://moz.com/blog/how-should-you-handle-expired-content
