Question : A Web part with this ID has already been added to this page

A page in our internet site suddenly became inaccessible and showed the above error. The page could not be opened in SPD or the browser, even using ?contents=1.  There are several pages in this site with DVWP referring to the same list, each web part is filtered to show a different type of list entry. Editing one of the working pages and changing the filter yielded the same error. Finally, making a copy of a working page and editing it did work.

This problem has now surfaced in our intranet - now 2 pages are affected. No changes have been made to the site or any pages within it for the last month at least.

Since the issue has appeared in two different web apps, I'm concerned that there is something going on at the server level.  Any help will be much appreciated.

Answer : A Web part with this ID has already been added to this page

A delegation is just a hint, that another DNS server is responsible to resolve a name.

If you have .kratos.us as Forest DNS zone, and several subdomains i.e. sub.kratos.us, then you can add a delegation on the DNS Server for .kratos.us within this zone for "Sub" which points to another DNS Server. This tells the requester, that the DNS Server in the domain sub.kratos.us is responsible for this (sub)domain.

If you do not have subdomains, there is not need for any delegations / DomainDNS Zones and ForestDNS zones are the same.
The last two settings are set by windows, if you install a dc into a forest. If all DNS are Active Directory integrated, there is usuall ynot need to add setting manually.