Hi, Denismtl!
I can't help but wonder if this is a homework question. Is it? Or, do you have another reason for asking, and if so what, please?
I like to use Google the most because I find it to be the most concise, accurate and up-to-date search engine. On Google, repeated words aren't included in the searches. However, on a search engine like Yahoo, repeated words are included. I have also found really old information on Yahoo that might not even be available anymore on the sites that it finds.
I'm not all that familiar with the other sites that you mention.
Hopefully, others will also come along to address your question.
Thanks!
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