I read somewhere that I can post the same blogs on 5 different blogging sites. Is it true? Will google consider it as duplicate content and promotion of website?
Yes google will consider this as content spamming and you should avoid doing such things. You can spin the articles into 5 different versions and then post it on 5 different blogging sites and in this case the content spamming issue would be reduced to a great extent.
As Per My Point of View you Can Post because Spreding content is not spamming activity so start posting this is the best way to generate traffic as well as good quality Back links... i am also offer guest blogging harendrasinghrajput.com this is my blog you can post here also if content is good...
Yes, it will be considered duplicate content. See, duplicate content is blocks of content that exactly match or are appreciably similar appearing on the Internet in more than one place (URL). Your blog may be 100% unique, agreed, but when this very unique blog is rolled across 5 sites, it ceases to remain unique. Best is to change it to such an extent that only the meaning remains the same.