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I would not post the same piece of content 15 times. I would post it just once on my highest authority website. I would then mention that it was posted on all the other 14 domains and link them to the piece of content.

Definetly Not, i would rather invest my time searching for potential niches that have exact searches of more than 6000 per month. cheers

Seems to be a significant difference between 5 x 1000 and the rest. So if you want to rank with that term you should definitefely set up an extra page for that and treat that indivually. 5 x 1000 can mean everything ... The lasting 5 terms are similar and we know that the search engines use synonymes, too - therefore we should definetifely use them. Althouth I made the experience that it is really difficult to rank with all synonyms. The best would be to set up a page for each term - what probably will be very difficult.