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:
(3) <meta name="keywords" content="keyword analysis, keyword optimization, analysis, optimization, keyword">
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">
source:
http://www.seologic.com/faq/keyword-optimization.php