At times, “we see that some webmasters try to compete for visibility in Google search results with a large number of sites on the same topic. There’re a few things to keep in mind when considering a strategy like this for sites that you want to have listed in our search results.”
“If all of your sites offer essentially same content, additional sites aren’t contributing much to the Internet. It’s a good idea to give each site its own content, personality and function.
A few webmasters choose to focus their resources on one domain but make use of their domain portfolio by creating a multitude of smaller sites linking to it. In some situations these sites may be perceived as doorways. Without value of their own, these doorway sites are unlikely to stand the test of time in our search results. If you registered several domains but only want to focus on one topic, we recommend you create unique and compelling content on each domain or simply 301 redirect all users to your preferred domain.
We suggest not spreading out your efforts too broadly, though. It can be difficult to maintain multiple sites while keeping the content fresh and engaging. It’s better to have one or a few good sites than a multitude of shallow, low value-add sites,” Google Search Quality Team.