Merge multivalue field then remove duplicated nodes

Question

I'm open to suggestions as how to fix the following:

• Drupal 7 website, I created a content with a multivalue field (checkboxes).
• I imported data from a csv file using feeds. Over 20K records were imported, with duplicated rows, except for the multivalue field.
• Feeds importer does not appends the multivalue field, instead it overwrites the value. It was the reason I decided to import all rows including duplicates.
• I created a unique key, using computed field, by concatenating some of the text fields that were common to each record. This key might be useful to consolidate the nodes later on.

How can I iterate through all the nodes, find the nodes with the same key, consolidate the values of the multivalue field, then delete the duplicates?

what about creating a temp content, same fields, to save the newly consolidated node? what is the easiest way to solve this problem?

Multivalue field (checkboxes)
A
B
C
etc

Content:

Key, NID, Title, Other fields, Multivalue field
key1, 1, title1, others1, A
key1, 2, title1, others1, B
key2, 3, title2, others2, A
key1, 4, title1, others1, C

expected results, consolidate the multivalue text field to have a single node: NID can be generated during the temp node creation.

NID, Title, Other fields, Multivalue field (checkboxes checked)
x, title1, others1, "A, B, C"
x, title2, others2, A

Answer 1

I finally figured out how to remove duplicates, while importing data with feeds, and using rules to consolidate the multi-value field. The following video was very useful to get to the final solution:

https://vimeo.com/28659739

Solution (based on custom needs):

1. create a key for each record in the csv file (I had many duplicates and no keys, data was not consolidated).
2. create a temporary field (radio buttons, one value), the field was a Term reference field.
3. import csv file, utf8 encoded, using the key as unique value (field_validation module).

Settings for Node processor

• Insert new nodes
• Update existing nodes
• Skip non-existent nodes

Importer updated the nodes with the same key, eliminating duplicates and consolidating records with the same information.

A Rule was executed, after updating a node, to add values to the multi-value field.

**Events**
After updating existing content of type Course
Before saving content of type Course

**Conditions**
Data comparison
Selected data: node:field-temp
value: equals
the value to compare

**Actions** 
Fetch entity by id
Value: taxonomy term
value: term id of the term you want to add to the multivalue field
Variable label: term name here
Variable name: custom_var_name

Add an item to a list
Selected list: node:field-multivaluefield
Data selector: custom_var_name
Enforce uniqueness: True.

Save entity
node:field-multivaluefield:0