Andy,
YES, repetition of singletons is allowed—even needed—with the keywords metatag—to the extent that you don't repeat too many times.
So let us rephrase—and give an answer to—your question in a more generic way: if the main keyword combination is "A B C" then the meta keyword tag could be "A B C, B C, B, D C, E C", for instance. New singletons—like "D" and "E" in our example—introduce diversity into the keyword list while keeping them specific and related/relevant to the main keyword phrase.
And even though some SE's—among which Google—reportedly don't take meta keywords into consideration, the mere exercise will definitively give you good ideas about topics to write content about. So just for that, it's very useful—and should be the **first** thing any content author should do.
===== Additional Information (you may know already) =====
Let me show you how simple it is in definitive to better it—just a little bit...
So to obtain that keyword richness, you'd better be as specific as possible by grabbing within your content the longest keyword combinations—and as many as possible—including the stem, and sorting them out by descending order of importance (i.e., the ones used most often)
So this was the quantitative part. Now here is the qualitative part.
Now by breaking them into singletons and sorting them alphabetically, we get a list that helps us count the words and duplicates. The rule to follow is 20-30 singletons including repetitions, no more than 3 repetitions of the same singleton—given all forms of the same words (e.g., singular, plural, gerund, infinitive) are considered differents keywords. Theses rules may vary overtime significantly—they did actually from the very begining of my career—and that's what I'm using nowadays.
So you'll better benefit from such methodology to help you craft your metakeywords.
Furthermore, you'll see that starting with this process to create a page will give you some very valuable hints about what to write around the theme you had in mind initially. This will also give you ideas for other keyword combinations for which you want to have a special treatment by dedicating them a specific—and optimized—content page.
Regardless of the fact that SE's reportedly give less and less weight to the meta keywords, benefits for you are that you know now what to write about to give a chance to SE's to better qualify your pages—and you'll see over time that SE's will start sending traffic for less or otherwise specific combinations like "making money".
This way, you get all the chances on your side to receive traffic that is better qualified for what you have to offer in terms or value, hence a better conversion of visitors into users or customers, in few words, a better return on investment (ROI).
And this recognition (i.e., more people clicking on your page) is another parameter that—aside from PR and relevancy—is taken into account by SE's to determine the best results to return. A catch-22.
Hope you're able to see now the differences in both quantitative and qualitative terms from the keywords you proposed initially in your question ;-)
In terms of tools, Google Keyword Tool (it's free) is good to start investigating potential keyword combinations:
https://adwords.google.com/select/KeywordToolExternal
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Pascal