We had a similar issue when we went from an old domain to our new domain last year in July. It took Google about 2 months to start recognizing our new domain and all the pages on it. During the first couple of months, we saw search results for pages with our new and old domain names.
Here is what we did to mitigate the issue:
-
Double check that the change of address tool was used correctly and that Google was aware of our new domain.
-
Redirect the old domain to the new one.
-
Monitor Webmaster tools for 404 errors and redirect links accordingly as they came up.
After 2 months and at this point, we can’t find any traces of our old domain on search results.
Good luck!
Vic