Does multiple same keywords in meta description cause keyword stuffing? ie.
<meta name="description" content="I love milk, milk makes me laugh all the time, laugh milk seems to be the most lovable milk, milk laughs at me and makes me love milk" />
All that does is hurt you even if it isn't keyword stuffing.
So to summarize, don't do it. It's pointless.
Your meta description should simply be a short description of what your page is about. It typically is displayed on the SERP (Search Engine Result Page), although lately, Google is more likely to show whatever content from the page is relevant to the user search query, as opposed to the meta description.
With that being said, it's still useful, and beneficial, to have custom meta descriptions for every unique page of your site, or landing pages that you want to target. Don't keyword stuff them. Don't overthink the keyword thing. Use keywords naturally. Google has gotten smarter, and knows synonyms of words. If you keyword stuff your meta descriptions, I can't see it at all helping you rank. Put your efforts instead to writing a short concise meta description that will tell users (and the search engines) what the page is about.
For example:
Milk is great for your body and overall health. Find out the nutritious benefits of drinking a glass of milk a day.
Or something like that.
Sidenote: Mine would be more like, "Find out the truth about the hormones and additives put into the Milk we used to believe was healthy for us" LOL - I only say this because as I was writing that mini sample above, it felt slightly hypocritical to me. Please don't take any offense at all. I'm actually upset I can't any longer drink much milk, I do miss it!
Also to add onto another response above, I second that the meta description isn't a ranking signal itself. It does help click through rate, and I do personally believe that Google looks at the overall optimization of the site (like the number of pages with custom, relevant meta descriptions), as a tiny itty bitty factor of your optimization, but I would re-inforce that view, that this wouldn't have any positive impact, essentially for that reason as well, Google doesn't use it itself as a signal.