You can run any UA-XXXXX-YY on any website (any domain name on any IP address), it will be completed in your profile statistics.
Yes, your statistics can be completely confused if any site puts your UA-identifier on your site.
To prevent this, you can configure the inclusion filter on your GA profiles and include only your domain names using regep like this: "www.domain1.com | www.domain2.com | www.domain2.com" in the host name field.
Be careful, the Include filter is exclusive (if you have 3 Enable filters in the hostname field, only the first will be applied).
An alternative is to create an extended segment based on the host name, you will get basically the same result, but may be subject to the effects of data sampling if you have a large audience. But this allows you to quickly fix it later, while filters must be installed before you run into problems.
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