There are two questions here, one asked and one unasked. First, we'll address the question about whether to use individual keywords or keyword phrases in your meta tags. Then we'll address the unasked question, "Do I have a chance to win for either 'historical research' or 'historical writing' with a page about both historical research and historical writing?"
Keyword Optimization: Individual Words or Phrases?
Consider two pages which simply feature large images, for example, pictures of two charts. The first page has the following meta keywords tag :
(1) <meta name="keywords" content="keyword analysis, keyword optimization"> The second page has the following meta keywords tag :
(2) <meta name="keywords" content="keyword, analysis, optimization"> If you search using the keyword phrase "keyword analysis" with and without the quotation marks, the first page will rank higher than the second page every time, all else being equal. The same goes for the keyword phrase "keyword optimization" and for the individual keyword, "keyword." On the other hand, the second page will win for the keyword searches "analysis" and "optimization." Now consider a third page with a picture of another chart. The third page has the following meta keywords tag :
With the addition of this third page, the results for our competition are shifted. Now the third page, which includes the words and phrases, wins for every single search, including keyword analysis, keyword optimization, keyword, analysis, and optimization.
The answer to the original question above is yes, kind of. You should duplicate words in similar phrases, but you should also enter in the individual keywords. The final optimized keywords list for the original question would be: <meta name="keywords" content="historical research, historical writing, research, writing, historical">
The unasked question: "Do I have a chance to win for either 'historical research' or 'historical writing' with a page about both historical research and historical writing?"
From your question, it is clear that you are a writer who is offering historical research and writing services, most likely for large companies, and that you hope to use the Internet to generate leads.
Generically speaking, when building your website, you should build a page about each keyword phrase you hope to win. In your case, you need a page about your historical research services, and a page about your historical writing services.
Unfortunately, "historical research" is an incredibly competitive phrase, and you have little chance of ever getting a top ten ranking for that phrase without expending an inordinate amount of time and energy. You have a much better chance of winning for "historical writing, " but even that phrase is very competitive.
We suggest you focus on "historical writing" and "historical writer" as your primary keyword phrases, and then do Pay-Per-Click marketing for your "historical research" phrase. At the time of this writing, you can buy a top 3 Overture listing for the phrase for $0.11/click. Actually, you can buy a top 3 ranking for "historical writing" for just $0.05/click as well, which would supplement your SEO efforts nicely.