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So, domain1.com, domain2.com and domain3.com all point to maindomain.com. Domain1.com and domain2.com were aliased about 8 months ago, domain3.com about 3 months ago.
These were set up as domain aliases through our hosting company.
While checking/searching for anyone stealing our content, I noticed that Google was listing domain1.com AND maindomain.com as having the same content? (duplicate pages - same page names, everything... except the main domain name, obviously)
Even worse, in WMT I noticed when checking my EXTERNAL links, it showed domain1.com, domain2.com, and domain3.com ALL as being external sites linking to maindomain.com.
This may explain our major ranking drop in the past few days?
#*$! did I do wrong - how do I fix it? Please help.
it showed domain1.com, domain2.com, and domain3.com ALL as being external sites linking to maindomain.com.
They are external to your main domain - they have a different domain name.
Sure enough, the domain aliases were not done with 301 redirects. I implemented proper domain management, set up the 301's, and sent in a reinclusion request explaining the situation.
Time will tell - I'll update if anything happens.