I'm installing a beta package into a Spring '13 Sandbox org.
Previously we didn't have any issues with installing into this sandbox, but now I am getting an error, which our partner support has revealed to us is:
Object.CustomExternalIdField__c: duplicate value found: %3cunknown%3e duplicates value on record with id: %3cunknown%3e:
I've changed the name of my object/field for privacy sake, but this is the only reference we have to the object in our source code. Nothing changed about it between this and any previous packaged versions.