What Is A Proper Keyword String?
 

Is it bad to use the same key phrases or words over and over in each string or is each string considered as a separate keyword or phrase?

It is keyword stuffing and bad practice for SEO.

The terms should reflect ideas essential to your research topic. Use multiple terms to narrow your search. For example, to find information on the ability of Bell operating companies to compete in long-distance markets, you might use these terms and phrases:
Bell, telephone, long-distance, competition

Hi
repetition of keyword is harmful for a website.repeating the same keyword over and over again, to the point where it is making it hard for Google to "recognize what's really unique & compelling" on that specific site.

yes you should use some different keyword all the time. you should use keywords which are most often used by users on the search engines generally users use some misspelled key words as well so you should also include some commonly misspelled keywords as well.

A keyword string is a set of descriptive words that you use to identify a problem. A keyword is a word or abbreviation that describes one part of a program failure.

Now a days more stress is laid on long tail keyword. Gone are the days when keyword stuffing, keyword density were been given utmost weightage.

Long tail keywords are really useful, and liked by Google and other search engines.

Yes it is bad to use same keyword phrase over and over as it will be considered Black Hat SEO technique and it will harm your site. Use different keywords for your article or content and keep distance between them.

you should go for different keywords or you can use a long tail keyword with targeting area. A long tail keyword is more preferable than 2 to 3 short tail keywords. Because long tail keyword is considered as a search query entered in the search engine, so that google could easily find the proper content for his users.

It is important to use some different keyword sometimes. Try to use the ones that are most frequently used by users so that it is helpful in attracting traffic.

Thanx friends for your great suggestions.

Try to use those keyword , which is frequently used by users.

Using various keywords is important in a website. Make good use of keywords used or searched by users to attract more traffic.