Open "All Messages/Received" and click the "settings for this view" icon on the message list toolbar. Make sure "show hidden" is unchecked". There's a "show duplicates" there too, but in this case, you want to uncheck "show hidden" so the copy from "Inbox" (and the copies from other *Gmail* labels) don't show. The only one that should show in "All Messages/Received" is the copy from "[Gmail]/All Mail".
If unchecking "Show hidden" doesn't help, that most likely means the flags in index.ini in the "mail" folder for the "Inbox" IMAP folder (and any Gmail Label IMAP folders) are not set right. As in, the "hide from other" bit isn't set for those folders.
Now, if this is with sent messages only, you have to create an Opera label with a a From rule that matches the account. Then, you need to set the label to "hide these messages from other views" in the label's properties. Then, as long as "Show hidden" is unchcked, you should see extra copies of sent messages.
This stuff is only related to Gmail IMAP as Gmail uses a separate copy of a message for each label the message is in (Inbox is a label). The copy in [Gmail]/All Mail that "All Messages/Received" shows is the original copy.