You have able and present moderators that have the tools to decide if a user is voting on his other accounts with the intent of inflating his reputation.
Voting, accusing a user of sock-puppeting, is not and will not - imo - be of any use for normal users and I refuse to imagine how can that be of any good for the site.
There is no rule that says a user ...
Regarding Duplicate Questions:
When you find a duplicate question, you should flag one of the question as a Duplicate of the other. If you have the privilege to Vote-To-Close, you should exercise your privilege as such.
Which one should be flagged/VTC'd?
The one which is poorly asked, is poorly formatted, has poor grammar, is lacking in information and so ...
Can you still access your older account? If so, head to the "Merge User Profiles" page, and follow the instructions.
If that doesn't work, you can submit this request to Stack Exchange from this page: https://askubuntu.com/help/other
As Seth said you can use multiple login methods on StackExchange, one of them is OpenID and for example the link to your Launchpad profile. Launchpad and StackExchange however are completely independent and owned by different entities, they are not different products from one company where an employee can merge your accounts.
To help you with the question on why weren't all askubuntu accounts automatically linked to stackexchange, it is because, back when askubuntu was born, the stackexchange worked on common account systems (google, facebook) and their own stackexchange system. The launchpad for what I remember (Anyone please correct me if am wrong) was added later, but both ...