UCM has got inbuilt features DeDuplication and Survivorship Rules which govern / evaluate the matching percentage of 2 given records (DeDuplication) and update of certain attributes on the survivor post merge (Survivorship Rules). However if you have integration of the standardized data back into Siebel, you would be required to maintain the same in Siebel. This is where a custom merge solution comes handy.
To build a custom merge solution the following pointers should be kept in mind:
a. Merge of 2 records (Way to identify the survivor and victim records)
b. Update of attributes / fields from victim onto Survivor record (Survivorship rules)

The solution would be a 3 step process (high level though)
1. Ensure you have ways to identify the Victim and Survivor records based on merge performed in UCM
2. Capture the attributes / fields from the victim record which you would like to update on the survivor record (based on Survivorship rules in UCM)
3. Perform OOB Merge on the records
4. Explicitly update the fields on the survivor record per pointer 2.

I will update the group on other questions in sometime related to Foreign key reference and child record association / disassociation resolution.

Apart from above pointers, if Data Management application like UCM is in use, there is a requirement to merge the records with certain attributes of the victim record to be retained on the survivor record.